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Intra-Transform Volcanism along the Siqueiros Fracture Zone 8 Degrees 20 Minutes N to 8 Degrees 30 Minutes N, East Pacif...


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INTRA-TRANSFORM VOLCANISM ALONG THE SIQUEIROS FRACTURE ZONE 8 20’ N – 8 30’ N, EAST PACIFIC RISE By MICHELLE RENAE HAYS A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2004

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ii ACKNOWLEDGMENTS Foremost, I thank Dr. Michael Perfit fo r all his guidance a nd support throughout this project. I thank my committee members, Dr. Paul Mueller and Dr. David Foster, for their time and discussion. I give special thanks to Dr. Daniel Fornar i, Dr. Ian Ridley, the captain and crew of the Atlant is II, and the Alvin pilots fo r this project would not have been possible without their effo rts. I especially thank Dr Daniel Fornari and Dr. Ken Simms for their early guidance in my gr aduate studies. I thank Dr. Leonard Danyushevsky for the Cameca microprobe analysis completed at the University of Tasmania, Dr. Ian Jonasson for the ICP-ASE da ta from the Geological Survey of Canada, and Dr. Jack Casey of the Univ ersity of Houston for ICP-MS analysis. I would also like to thank Dr. Robert Shuster and Dr. Harmon Ma her from the University of Nebraska for their encouragement and inspirati on. Most of all, I thank Troy and the rest of my family for their love, support and everlasting patience. I could not have made it this far without them. This work was supported by The National Science Foundation through grants OCE-90-19154, OCE-90-20404, and OCE-0138088 a nd by the University of Florida through the Alumni Fellowship.

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iii TABLE OF CONTENTS page ACKNOWLEDGMENTS..................................................................................................ii LIST OF TABLES...............................................................................................................v LIST OF FIGURES..........................................................................................................vii ABSTRACT......................................................................................................................x ii CHAPTER 1 INTRODUCTION........................................................................................................1 Sampling of the Siqueiros Transform...........................................................................3 The Transform Fault Effect..........................................................................................6 Mantle Heterogenities...................................................................................................8 2 GEOLOGICAL RELATIONSHIPS AND SAMPLE LOCALITIES IN THE SIQUEIROS TRANSFORM......................................................................................10 3 ANALYTICAL METHODS......................................................................................20 4 PETROGRAPHY AND LOCAL GEOLOGIC RELATIONSHIPS..........................28 Samples from the A-B Fault.......................................................................................28 Siqueiros Sample Petrography....................................................................................42 Crystal Liquid Equilibria............................................................................................44 5 MAJOR AND TRACE ELEMENT CHEMISTRY...................................................61 Major Element Trends................................................................................................61 Comparison of Siqueiros Samples to th e Adjacent EPR and Garrett Transform.......73 Trace Element Trends.................................................................................................79 6 PETROGENESIS.....................................................................................................104 Major Element Models.............................................................................................104 Trace Element Models..............................................................................................119 REE Models..............................................................................................................133

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iv 7 DISCUSSION...........................................................................................................138 Fractional Crystallization..........................................................................................138 Magma Mixing and Assimilation.............................................................................140 D-MORBs and E-MORBs........................................................................................148 Controls on Spatial Variabil ity in Lava Chemistry..................................................150 Tectonic Controls on Magmagenes is and Melting Systematics........................156 Constraints on Melting –Na-Fe Systematics.....................................................159 Models for Volcanism in Transform Domains.........................................................168 Garrett Transform Models.................................................................................170 Siqueiros Transform Models.............................................................................174 Proposed Model.................................................................................................176 8 CONCLUSIONS......................................................................................................180 APPENDIX A NORMALIZATION OF CAMECA MICROPROBE DATA.................................182 B OLIVINE, PLAGIOCLASE AND SPINEL MICROPROBE ANALYSIS.............185 C MAJOR ELEMENT COMPOSITIONS OF THE SIQUEIROS SAMPLES...........199 D TRACE ELEMENT CONTENTS OF THE SIQUEIROS SAMPLES....................209 E FRACTIONAL CRYSTALLIZA TION MODEL PARAMETERS CALCULATED IN PETROLOG.............................................................................223 F REGRESSION ANALYSIS FOR FE8.0 AND NA8.0...............................................237 G LIST OF REFERENCES..........................................................................................241 H BIIOGRAPHICAL SKETCH...................................................................................251

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v LIST OF TABLES Table page 2-1 Siqueiros transform Alvin dive locations.................................................................15 2-2 Siqueiros transform dredge locations.......................................................................16 4-1 Thin section descriptions..........................................................................................29 5-1 Nb, Sr, Zr, and Y enrichment factor s for Siqueiros transform morphotectonic locations...................................................................................................................86 6-1 List of partition coefficients...................................................................................120 6-2 REE partition coefficients......................................................................................134 B-1 Microprobe analysis of olivine phenocrysts in the Siqueiros samples...................186 B-2 Microprobe analysis of plagioclase phenocrysts in the Siqueiros samples............190 B-3 Microprobe analysis of spinel phe nocrysts in the Siqueiros samples....................195 C-1 ARL, JEOL, and DCP electron microprobe major element analyses of basalts from the Siqueiros transform..................................................................................200 C-2 Siqueiros glass major element analysis..................................................................205 D-1 XRF trace element concentrations for the Siqueiros transform basalts.................210 D-2 ICP Trace element concentrations for the Siqueiros transform basalts..................214 D-3 DCP trace element concentrations for the Siqueiros transform basalts.................217 D-4 ICP trace element concentrations of the Siqueiros transform basalts....................218 E-1 2377-7P at low pressure.........................................................................................224 E-2 D34-2P at low pressure..........................................................................................225 E-3 2384-9P at low pressure.........................................................................................226 E-4 2384-9P at 2 kbar...................................................................................................227

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vi E-5 2377-7P at low pressure, hydrous conditions.........................................................228 E-6 D34-2P at low pressure, hydrous conditions..........................................................229 E-7 2384-9P at low pressure, hydrous conditions.........................................................230 E-8 2377-7P at low pressure using fract ionation model of Langmuir (1992)..............231 E-9 D34-2P at low pressure using fr actionation model of Langmuir (1992)...............232 E-10 2384-9P at low pressure using fract ionation models of Langmuir (1992).............233 E-11 2384-9P at 2 kbar using fractiona tion models of Langmuir (1992).......................234 E-12 D20-15P at low pressure........................................................................................235 E-13 2375-7P at low pressure.........................................................................................236

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vii LIST OF FIGURES Figure page 1-1 Location map of Siqueiros transform.........................................................................2 1-2 Sample location map..................................................................................................5 2-1 Plate boundary geometry of the Siqueiros transform...............................................11 2-2 Bathymetry and sample locations for west side of the Siqueiros transform............18 2-3 Bathymetry and sample locations for east side of the Siqueiros transform.............19 3-1 Graphical comparison of ARL micr oprobe, JEOL microprobe, DCP, and Cameca SX50 data before correction of the data.....................................................23 3-2 Comparison of data after adjust ment of the Cameca SX50 MgO and P2O5 contents.....................................................................................................................25 4-1 Photomicrographs taken under plain light (a & b) a nd cross polarized light (c & d)......................................................................................................................36 4-2 Photomicrographs taken under plain light (a, b, & d) and cross polarized light (c)............................................................................................................................ .37 4-3 Photomicrographs taken under plain light (c) and cross polarized light (a, b, & d).................................................................................................................38 4-4 Dredge and Alvin dive loca tions within the A-B fault.............................................40 4-5 Alvin dive 2384 traverse..........................................................................................41 4-6 Comparison of olivine forste rite content with the Mg# (Mg2+/(Mg2+ + Fe2+)) of the host glass........................................................................................................45 4-7 Comparison of Olivine forsterite content with the Mg# (Mg2+/(Mg2+ + Fe2+)) of the host glass........................................................................................................47 4-8 Calculated Fo contents of olivine for partition coefficients ranging from 0.28 to 0.32........................................................................................................................... 48

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viii 4-9 An contents for core, interior, and rim locations in Si queiros plagioclase phenocrysts...............................................................................................................49 4-10 Comparison of plagioclase An content from Siqueiros samples and An content evolution for three of the major element parental compositions..............................50 4-11 Comparison of the host glass Ca# (100*C a/(Ca + Na) with the plagioclase An content. ...................................................................................................................52 4-12 Comparison of Siqueiros plagio clase An content vs. glass Ca# (100*Ca/(Ca +Na)). ................................................................................................53 4-13 Spinel Cr# for core, in terior, and rim locations........................................................54 4-14 Fe3+/(Cr + Al + Fe3+) vs. Fe2+/(Mg + Fe2+) plots for tholeiitic basalts.....................55 4-15 Cr/(Cr + Al) vs. Fe2+/(Mg + Fe2+) plot for tholeiitic basalts....................................56 4-16 Molecular percentage aluminum in gla ss versus molecular percentage aluminum in spinel....................................................................................................................58 4-17 Comparison of the composition of the co res, interiors, and rims of spinels found in the groundmasses and within oliv ines with the composition of the host glass.......................................................................................................................... 58 4-18 Comparison of the composition of the co res, interiors, and rims of spinels found in the groundmasses and within oliv ines with the composition of the host glass.......................................................................................................................... 59 4-19 Comparison of the composition of the sp inels found inside olivines and spinels found in the glass with the composition of the host glass........................................59 4-20 Comparison of the composition of the sp inels found inside olivines and spinels found in the glass with the composition of the host glass........................................60 5-1 Major element variation diagrams for glasses from the Siqueiros transform domain......................................................................................................................62 5-2 Major element variation diagrams show ing the Siqueiros picrites and picritic basalts relative to more evolved MORB as in Figure 5-1........................................65 5-3 Comparison of K2O/TiO2 of Siqueiros samples with samples from the EPR..........69 5-4 MgO (wt. %) and depth to seafloor vers us distance from the axis of spreading center B....................................................................................................................74 5-5 Variation diagrams comparing Siqueiro s lava compositions w ith basalts from the 9-10N segment of the EPR...............................................................................75

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ix 5-6 Variation diagrams comparing the co mpositions of the Siqueiros and Garrett samples.....................................................................................................................77 5-7 Trace elements versus TiO2......................................................................................80 5-8 Trace elements versus Zr..........................................................................................83 5-9 Ce/Ybn vs. K2O/TiO2 of the Siqueiros samples.......................................................88 5-10 Chondrite normalized Ce/Yb ratios fo r Siqueiros morphotectonic locations..........89 5-11 Chondrite normalized REE diagrams.......................................................................91 5-12 N-Morb normalized REE diagrams..........................................................................95 5-13 N-MORB normalized Ce/Y ratios fo r Siqueiros transform morphotectonic locations.................................................................................................................100 5-14 REE diagram of RTI E-MORBs pl otted relative to E-MORB values...................100 5-15 Primitive mantle-normalized trace element diagrams............................................101 6-1 Percentage of crystals remove d as a function of temperature................................106 6-2 Percentage of liquid and removed crys tals as a function of percentage of crystals removed from magma for 2377P, D34-2P and 2384-9P..........................107 6-3 Comparison of major element data w ith LLD models calc ulated using the olivine, plagioclase, and clinopy roxene fractionation models of Danyushevsky........................................................................................................109 6-4 Comparison of major element data w ith hydrous LLD models calculated using the olivine, plagioclase and clinopy roxene fractionation models of Danyushevsky........................................................................................................113 6-5 Comparison of CaO and Al2O3 data with LLD models calculated using the olivine, plagioclase, and c linopyroxene fractionation mode ls of Langmuir et al..118 6-6 Comparison of observed trace element data with modeled fractionation trends calculated assuming perfect Raylei gh fractional crystallization............................121 6-7 Comparison of observed trace element data versus TiO2 with modeled fractionation trends calculated a ssuming perfect Rayleigh fractional crystallization.........................................................................................................124 6-8 Comparison of observed trace element data versus TiO2 with modeled fractionation trends calculated a ssuming perfect Rayleigh fractional crystallization.........................................................................................................129

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x 6-9 Comparison of observed trace element data versus Zr with modeled fractionation trends calculated a ssuming perfect Rayleigh fractional crystallization.........................................................................................................130 6-10 Comparison of observed REE trends with modeled REE fractionation trends calculated for 2375-7P from spreading center B....................................................134 6-11 Comparison of observed REE trends with modeled REE fractionation trends calculated for D20-15P from the A-B fault............................................................135 6-12 Comparison of observed A-B fault REE trends with modeled REE fractionation trends calculated for D20-15P................................................................................135 6-13 Rayleigh fractionation model for REE...................................................................137 7-1 Mixing lines between primitive and evolved sample compositions from the Siqueiros transform................................................................................................144 7-2 Trace element mixing lines between primitive and evolved samples....................146 7-3 Calculated mixing curves between sa mple 2384-9 and an evolved sample from spreading center B (2377-11) and an E-MORB from the RTI (2390-1)................147 7-4 Chondrite and N-MORB normalized Ce /Y ratios for Siqueiros transform morphotectonic locations.......................................................................................149 7-5 Chondrite normalized La/Sm ratios fo r Siqueiros transform morphotectonic locations.................................................................................................................151 7-6 Calculated mixing between sample D20-15 (D-MORB compositions) and sample 2390-1 (E-MORB composition)................................................................152 7-7 LLD for 2384-9P after mixing with 10% E-MORB..............................................153 7-8 Modeled fractional crystallization path of 6% mixing line from figure 7-6. .......154 7-9 Location map of E-MORB, N-MORB, and D-MORB samples within the Siqueiros transform based on Ce/Y ratios..............................................................155 7-10 Position of “apparent” Euler poles asso ciated with a counterclockwise change in spreading direction along the Clippe rton and Siqueiros Fracture Zones...........157 7-11 Siqueiros Na8.0 and Fe8.0 data compared with global field for normal ridge segments.................................................................................................................162 7-12 Na8.0 vs. Fe8.0. .......................................................................................................163 7-13 Na8.0 vs. Fe8.0 and K2O/TiO2..................................................................................165

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xi 7-14 Ce/Y ratios vs. Na8.0 values for all Siqueiros transform samples...........................166 7-15 Ce/Y ratios vs. Na8.0 values for samples from spreading center A. ......................166 7-16 K2O/TiO2 ratios of Siqueiros samples compared with their Na8.0, Fe8.0 data.........167 7-17 Fe8.0 versus Na8.0 and axial depth...........................................................................167 7-18 Variations in Na8.0 and Fe8.0 systematics due to variab le depths and extents of melting....................................................................................................................168 7-19 Sample density versus recovery depth for Siqueiros samples................................171 7-20 Sample MgO content versus rec overy depth for Siqueiros samples......................171 7-21 Density of samples vs. depth with addition of olivine phenocrysts.......................172 7-22 Density vs. depth of Siqueiros samples with 5 modal % olivin e added to picritic and olivine rich basalts...........................................................................................172 7-23 Magma transport within the Siqueiros transform...................................................179 A-1 Cameca microprobe data versus ARL and JEOL microprobe data........................183 A-2 Normalized Cameca microprobe data. .................................................................184 F-1 Linear regression of Na2O......................................................................................239 F-2 Linear regression of FeO........................................................................................239 F-3 Polynomial regression of Na2O. ...........................................................................240 F-4 Polynomial regression of FeO................................................................................240

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xii Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science INTRA-TRANSFORM VOLCANISM ALONG THE SIQUEIROS FRACTURE ZONE 8 20’ N – 8 30’ N, EAST PACIFIC RISE By Michelle Renae Hays December 2004 Chair: Michael Perfit Major Department: Geological Sciences Detailed sampling and sonar mapping of the Siqueiros transform were completed in 1991 during the AtlantisII 125-25 Research Cruise. Fresh, glassy, volcanic rocks were recovered from small constructional volcanic la ndforms within leaky transform faults and from troughs within the transform. Three of the troughs within the transform exhibit organized spreading and are be lieved to be intra-transform spreading centers that have resulted from changes in the relative moti ons of the Pacific and Cocos plates. The samples recovered include extremely primitive lavas (pricritic and olivine-phyric basalts to high-MgO basalts). Compared to the adjacent 9-10N segment of the EPR the Siqueiros basalts are more primitive and tend to group on the more depleted end in major and trace element diagrams. Four chemica lly distinct groups of lavas have been identified within the transform. The spr eading centers have er upted only N-MORB type lavas which are similar to those from the EPR. Lavas recovered from shear zones within the transform tend to be more primitive and depleted in incompatible elements with the

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xiii most incompatible element depleted lavas (D -MORB) recovered from the A-B fault, the shear zone connecting the tw o western most spreading centers. The E-MORB samples were only recovered at the western ridge-transform inte rsection (WRTI) and a group of low Na2O samples were recovered within spreadi ng center A. Fracti onal crystallization models indicate that the majority of th e N-MORB samples can be explained by 50-60% fractional crystallization of olivine spinel + plagioclase + clinopyroxene of 2-3 parental compositions similar to the high-MgO lavas reco vered from the A-B fault. Scatter about CaO vs. Al2O3 trends and ratios among highly incompatible elements, along with variations in phenocrysts compositions, i ndicate that mixing between primitive and evolved compositions is needed in order to explain the entire range of major and trace element variations. Resorbtion textures and chemical analysis of many the large phenocrysts show they are out of equilibri um with the host magma and were derived from high CaO, high MgO lavas. REE diag rams show that the D-MORB samples from the A-B fault cannot be related to the NMORB samples by fractional crystallization alone. Mixing models indicate that N-MO RB compositions can be produced by mixing of approximately 4-6% of an E-MORB co mposition with the D-MORB samples. The low Na2O samples from spreading center A are best explained by mixing with a more depleted source, but Na8.0 and Fe8.0 data indicate both mixing of sources and variable extents and depths of melting occur within the transform. The compositional variations of the Siqueiros samples can be explaine d by a petrogenetic model in which lava compositions are controlled by the presence /absence, size, and depth of melt lenses within the transform.

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1 CHAPTER 1 INTRODUCTION The Siqueiros transform fault is a left lateral transform fault located on the Northern East Pacific Rise (NEPR) between 8 20’N and 8 30’N (Figure 1-1). The transform domain is approximately 20 km wide and offsets the NEPR by 138 km (Fornari et al., 1989). It lies along a fast-spreading portion of the EPR with a half-slip rate of approximately 63 km Ma-1 (Fornari et al., 1989). In 1991, detailed observational data and extensive sampling revealed 3 intra-transf orm spreading centers and small eruptive centers within the transform sh ear zones, all of which exhib it recent volcanism (Perfit et al., 1996). Transforms faults, such as Siqueiros, theo retically parallel th e direction of plate motion and are conservative plate boundaries where no plate construc tion or destruction is thought to occur. Volcanism within the Si queiros transform is unus ual, but is believed to be the result of counterclockwise change s in the spreading di rection between the Pacific and Cocos plates. Ro tations in plate motions resulted in an extensional environment within the transform (Pockal ny et al., 1997). Petr ologic and morphologic data suggest that volcanism does occur with in other transform domains that exhibit extension, especially along fastand superf ast-spreading portions of the Mid-Ocean Ridge crest (Perfit et al., 1996; Fornari et al., 1989). Few of these transforms have been sampled or studied in any great detail (Hek inian et al, 1995; Wendt et al., 1999). A few samples have been analyzed from the Raitt transform along the Pacific-Antarctic spreading ridge (Castillo et al., 1988) and some samples have been recovered from the

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2 Blanco transform between the Juan de Fuca and Gorda Ridges (Embley & Wilson, 1992; Tierney, 2003). The Garrett transform on the s outhern EPR is the only oceanic transform where magmatism has been extensively studied and samples have been analyzed. The processes that formed the lavas erupted in th ese environments and their relations to the nearby ridges are still poorly understood. Figure 1-1. Location map of Siqueiros transform. Siqueiros Cli pp erton

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3 Because of their presumed colder therma l environment, intra-transform lavas are removed from the large magma chambers bene ath the ridge, in which large volumes of melt are mixed. The study of volcanic transf orms may provide new insights into the scale of mantle heterogeneitie s and the compositions of the de pleted and enriched mantle because such components may not be thoroughly mixed in areas removed from the larger magma chambers beneath ridge segments. Th e Siqueiros transform offers a unique look at three, small, focused spreading center s which are separate from the large magma chambers beneath the EPR, which are belie ved to be sites wher e different mantle components are mixed. The Siqueiros transform also contains samples of primitive compositions rarely found elsewhere in close proximity to evolved samples. The goal of this study is to gain a better understanding of the petrologic segmentation and magmatic processes beneat h the Siqueiros transform and to compare the basalts of the Siqueiros transform domain with those of the adjacent EPR and Garrett transform. Major and trace elem ent variations in concert wi th crystallization and mixing models have been used to estimate parental magmatic compositions. Phase chemical data and incompatible element ratios and variations have also been used to evaluate the histories of crystallization and mixing. Sampling of the Siqueiros Transform The study of fracture zones is important b ecause rocks that are believed to compose the lower oceanic crust and upper mantle (ga bbroic and ultramafic rocks) are commonly found within fracture zones and rarely f ound elsewhere in the ocean basins. The Siqueiros transform was origin ally investigated to comp lement the knowledge gained from the slow moving Fracture Zone A in th e FAMOUS area of the Mid-Atlantic Ridge (Detrick et al., 1973; Crane, 1976). The first near botto m geological and geophysical

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4 survey was conducted at the western intersecti on of the Siqueiros transform fault and the East Pacific Rise (EPR) using a Deep Tow Fish (Crane, 1976). Samples were also recovered from the western most Siqueiros transform and adjacent EPR by rock dredging (Crane, 1976; Batiza et al., 1977; Natland, 1989). Sampling revealed a broad range of rock types, which include enriched midocean ridge basalts (E-MORBS), normal midocean ridge basalts (N-MORBS), and high-MgO rocks, but a l ack of precise locations for the dredged samples made it difficult to interpret the geochemical data in this area of complex sea-floor structure (Natland, 1989). The acquisition of a Sea MARC II sonar survey in July 1987 (Fornari et al., 1989) and Alvin submersible dive observations in May-June 1991 (Fornari et al., 1991) has allo wed a better understanding of the seafloor structure. The petrologic, observational, and morphologic data from the 1987 and 1991 cruises revealed what appeared to be sites of intra-transform spr eading (Fornari et al., 1989; Fornari et al., 1991; Perfit et al., 1996). Four troughs labeled A, B, C and, D and five strike-slip faults were identified using the bathymet ric and side-looking sonar data (Fornari et al., 1989). During Alvin submersible dives, fres h-looking pillow lava flows and sheet flows were identified along the spreading ridges and eruptive centers were found in transform shear zones (Figure 1-2) (F ornari et al., 1991). Despite their location far from the north and south tip of the East Pacific Rise, the basalts recovered from three of the troughs (A, B, and C) still have re latively unaltered glassy rinds, which suggests that they have been recently erupted and do not originate from the spreading associated with the adjacent East Pacific Rise. The fo rth trough, D, was found to comprise an older volcanic terrain, which has been strongl y tectonized. Trough D does not contain any identifiable ridges and any spreading is beli eved to be focused along transform parallel

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5 N$Spreading Center AUA-B Fault$ TSpreading Center B#B-C Fault$Spreading Center C%C-D Fault#Trough D'W-RTISample Locations# $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ $ $ $ $ $ $ $ U U $ T $ T $ T $ T $ T $ T $ T $ T# # # ## #$ T $ T $ T $ T $ T $ T $ $ $ U U U UU U UUU U U U U U U $ $ $ $ # # # # # ## # # # #U U U U U U U U $ $ $ $ $ $$ '' ' ' ' U U UU U U U U $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T U U U U U U U U U U $ T $ T $ T $ T $ T $ T $ T $ T $T $ T $ T $ T $ T $ T $ T $ T $ T $ T U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U $ $ $ $ $ $ % %# #' $ $ $ $ $ $ % % $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ ' ' # # # Map Location -104 -104 -103 -103 8 8 Figure 1-2. Sample location map.

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6 lineaments (Fornari et al., 1991). Troughs A, B, and C are believed to be small intratransform spreading centers (F ornari et al., 1991) where orga nized spreading is occurring. The lavas are remarkably fresh with little se diment cover and thick glassy rinds. Small constructional volcanic landforms were found at small offsets within the strike-slip faults connecting the spreading centers. The samp les within the A-B fault were unusually mafic, olivine-rich basalts (P erfit et al., 1996). The olivine-ph yric basalts are referred to as picritic basalts. In 199 6, Perfit and others conducted a detailed study of the young picritic basalts and high-MgO lavas from the AB fault. The picriti c basalts were found to be formed by the accumulation of olivine and minor spinel from high-MgO melts (Perfit et al., 1996). The high-MgO glasses recovered from the strike-slip fault were found to potentially be near-primary melts fr om incompatible-element depleted oceanic mantle that have been little modified by crustal mixing and or fractionation processes (Perfit et al., 1996). The Siqueiros samples collected in 1991 are pe trologically diverse and contain picritic basalts, ferrobasalts, Fe Ti basalts, N-MORB, incompatible element depleted normal mid-ocean ridge basalts (D-MORB) and E-MORB. This study will combine the previous work completed on the pi critic basalts from the A-B fault with a more detailed examination of the samples recovered from other localities within the transform domain in order to gain a bette r understanding of the petrologic and tectonic evolution of the Siqueiros transform. The Transform Fault Effect Geophysical studies indicate that an axia l magma chamber overlain by a thin melt lens is present beneath many sections of the EPR (Sinton & Detrick, 1992; Dunn et al., 2000). The seismic reflector representing the melt lens is <3 to 4 km wide and caps a low compressional wave velocity zone 5-7 km wide. This low velocity zone is believed to

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7 represent the presence of melt mixed with crystals to produce a “mush”. Near 930’N on the EPR the axial magma chamber reflector was found to be 1-2 km below the rise. Rosendahl et al. (1976) and Or cutt et al. (1976) demonstrated the existence of a crustal low-velocity zone about 4 km wide and 0.5 to 1.0 km beneath the crest of the EPR in the vicinity of the Siqueiros Fracture Zone. Although seismic studies have not been conducted beneath the Siqueiros fracture zone, seismic studies have shown that the axial magma chamber seismic reflector terminates ne ar fracture zones and is reduced at other discontinuities (Macdonald and Fox, 1988; Macdonald et al., 1991). The lack of a large magma chamber beneath transforms allows fo r the possible eruption of unmixed mantle components. Studies have shown that basalts adja cent to fractures zones tend to be characterized by more fractionated compositi ons (Melson and Thompson, 1971; Hekinian and Thompson, 1976; Natland and Melson, 1980; Christie and Sinton, 1981; LeRoex and Dick, 1981, Sinton et al., 1983; Fo rnari et al., 1983; Perfit and Fornari, 1983; Perfit et al.,1983; Langmuir and Bender, 1984; Elthon, 1988). At the Siqueiros ridge transform intersection (RTI), highly fractionated E-MORBs along with a few FeTi basalts have been recovered. A wide range of magma compositions includi ng highly–fractionated magmas have been found proximal to fract ure zones (Elthon, 1988). These observations have been explained by the “transform fault effect” in which the isotherms are suppressed near the transform due to the juxtaposition of older, cooler lithosphere agai nst the ridge transform intersection (Langmuir & Bender, 1984). The depressed isotherms may allow isolated magma pockets near the ridge transform intersection permitting the highly fractionated magmas to be devel oped (Perfit and Fornari, 1983).

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8 Mantle Heterogenities Initial studies along mid-ocean ridges found axial lavas to be rather homogeneous, but more intensive studies along strike have re vealed variations in basalt chemistry that are believed to relate to ridge segmen tation and morphology (Thompson et al., 1985; Langmuir and Bender, 1986; Smith et al., 1994; B azin et al., 2001). Transform faults are first order segments that part ition the ridge into distinctive tectonic units which persist for a million years or more and have been found to separate ridge segments with contrasting tectonic and petrological properties (Macdonald et al., 1988). Smaller second and third order segments, such as overlapping spreadi ng centers, deviations in axial linearity (DEVALS), small non-overlapping offsets (SNOOs), and kinks in the ridges, have also been found to correlate with geochemical segmentation (Langmuir and Bender, 1986; Bazin et al., 2001; Smith et al., 2001). The ax ial discontinuities can be related to the axial magma chamber’s depth beneath the seaf loor, width, thickness, continuity along the ridge, and the geochemistry of the er upted lavas (Macdonald, 1998). The axial discontinuities have also been found to be related to the volcanic segmentation of the ridge (White et al., 2002). Lava morphology (from sheet to pillow flows) has been found to coincide with boundaries of morphologically defined third-order tectonic segments of the ridge crest and to indicate reduced eruption rates (White et al., 2002). Studies of mantle heterogeneities have recently focused on across strike sampling in order to study the chemistry of off-axis eruptions, which may tap different sources without mixing in large magma chambers or mush zones. Studies on the East Pacific Rise (ERP) that have focused on across strike variations have found some off-axis lavas that appear to be younger than the surr ounding terrain and show greater chemical variability than axial lavas. Remarkably sma ll-scale spatial variations in basalt chemistry

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9 of these off-axis lavas have been found (Re ynolds et al., 1992; Perfit et al., 1994; Bideau and Hekinian, 1995; Perfit and Ch adwick, 1998; Castillo et al ., 2000). Detailed off-axis studies have revealed the existence of lavas with distinctive chemical compositions, both more and less enriched in incompatible elements than those delivered to the axis. Some are similar to depleted lavas recovered from near axis seamounts (Fornari et al., 1988; Perfit et al., 1994; Reynolds and Langmuir, 2000). A nonsystematic distribution of EMORB lavas was found off-axis in the 9-10 N region of the NEPR (Perfit et al., 1994, Perfit and Chadwick, 1998; Smith et al., 2001). It is believed that these resulted from frequent low-volume off-axis eruptions that did not reflect mixing within the large magma chamber beneath the ridge. Si gnificant chemical variation at 9 31’N was found to be on the scale of 200 m and is believed to result from both rapid changes in magma chamber chemistry and frequent low-volume on-ax is and off-axis eruptions (Perfit et al., 1994). Off-axis flows have been documented up to 4 km from the ridge axis along the EPR (Goldstein et al., 1994; Perfit and Chadwick, 1998; Sc houten et al., 1999; Reynolds and Langmuir, 2000). The source of these magmas is poorly understood. They may be fed by axial eruptions that flow great distances off-axis or they may be associated with an off-axis magma chamber. Transforms are also removed from the well-mixed large magma chambers associated with the ridge ax is. The intra-transf orm spreading centers can help provide a better understanding of the scale and composition of mantle heterogeneities and may be important in unde rstanding the source of off-axial eruptions.

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10 CHAPTER 2 GEOLOGICAL RELATIONSHIPS AND SA MPLE LOCALITIES IN THE SIQUEIROS TRANSFORM The Siqueiros transform is comprised of a number of different sections that are morphologically distinct (Figur e 2-1). The entire transform domain is about 20 km wide and includes the transform valley and adjacent seafloor that has been morphologically or structurally affected by proximity to the tran sform. Within the transform domain exsists the transform tectonized zone (TTZ) and the transform fault zone (TFZ). The TTZ is defined as the area truncated by abyssa l hill topography on oppos ite sides of the transform valley. The TFZ is usually a 2 km wide continuous swath of lineated ridges, troughs, and closed contoured basins. The Si queiros transform domain has been found to contain both shear and spreadi ng related features. The shear related features are a series of 5 en echelon TFZ which consist of ridges a nd troughs that nearly parallel the PacificCocos relative plate motions. The TFZ ar e approximately 15-25 km long and are the focus of recent strike-slip deformation. Th e fault troughs are deep (up to 3650 m) and narrow (1-3 km wide) especially in the wester n portion of the transform (Fornari et al., 1989). The spreading related features consis t of four extensiona l relay zones (ERZ), which are equivalent to continental pull-apar t basins. These ERZ are believed to have resulted from a series of counterclockwise chan ges in spreading direc tion that occurred at about 3.5, 2.5, 1.5, and 0.5 Ma (Pockalny et al., 1997). Very freshlooking lava flows and systematic aging of the seafloor across the axis and flanks were found at 3 of the relay zones during Alvin submersible dives c onfirming that there are three spreading

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11 Figure 2-1. Plate boundary geometry of the Si queiros transform. Locations of the 3 troughs that exhibit in tra-transform sprea ding (A, B, and C) and the fourth trough (D), wh ich has been strongly tect onized and does not exhibi t organized spreading are shown. Dashed lines show TFZ (A-B fault, B-C fault, C-D faul t) and the two faults that connect the spreading centers to the RTIs (WRTI-A and ERTI-D). Light shaded box depicts the transform domai n. Darker shaded boxes represent the WRTI and ERTI. Adjusted from Fornari et al., 1989. D

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12 centers (A, B, and C) within the transform. Th e fourth pull-apart basin (trough D) was strongly tectonized and did not exhibit any organized spre ading. The intra-transform spreading centers may have begun as leaky transforms that evolved into small welldeveloped spreading centers with the persistent change in th e plate geometry (Pockalny et al., 1997). The geologic locations referred to in this study include the western ridge transform intersection (WRTI), the eastern ridge transform intersection (ERTI), the 3 spreading centers (A, B, and C), trough D, and the TFZ. For this study, the transform faults were divided into offsets between the th ree spreading centers (A-B fault, B-C fault, and C-D fault). The A-WRTI and D-ERTI fault offsets were included as part of the WRTI and ERTI, respectively. At the ridge transform intersections (RTIs) the northern and s outhern limbs of the EPR axis become slightly deeper and sw ing into the transform domain, which is morphologically characteristic of transforms at the fast-end of the slip rate spectrum. Both the eastern ridge transform intersect ion (ERTI) and the western ridge transform intersection (WRTI) have unrifted crest and have abyssal hi ll topography characteristic of fast to medium spreading ridge segments. Spreading centers A and B both exhibit bilateral symmetry about the spreading axis out to 20-40 km. Spreading center A is the western most trough and is connected with the WRTI by a TFZ. It is a sigmoidshaped basin that consists of two ridges (Fornari et al., 1991). Lavas on the northern ridge are older and heavily overprinted. The southern portion has younger pillow lavas, but they are overprinted with faults and fissures, suggesting that transf orm tectonics are influencing th e area (Fornari et al., 1991). East of the southern A axial ridge the volcanic terrain is ol der and extensively weathered.

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13 West of the spreading center the transform fa ult intersects the EPR and lavas age to the north as the EPR is approached. Spreading center B is the most well developed spreading center with abyssal hill structures up to 8 km long. The youngest looking flows were found on a small 100 m cone near th e central portion of the axis. Spreading center B also consists of pillow walls and constructional pillow escarpments. Troughs C and D have much smaller swaths of abyssal hill to pography (10-20 km). Spreading fabric could only be identified within trough C. Fresh volcanics were found only within the floor of the graben and along the walls of the grabe n. Many of the flows within C are sheet flows. Trough D was found to contain only str ongly tectonized older volcanic terrain. Fresh basalts were recovere d north of D suggesti ng that any spreading at D is focused along a transform-paralle l lineament (Fornari et al., 1991). Transform faults A-B and B-C were chosen for detailed studies using ALVIN and the rock dredge because they link the most morphologically distinct and best organized intra-transform spreading centers. The fault zones are also very clear and have relief between 1000-1500m. Also, the axial deeps and RTI deeps are well-defined. The faults are approximately parallel (078 [A-B] and 075 [B-C]) to the relative plate motion of the Pacific and Cocos plate (082 ). Within the A-B transfor m young glassy picritic basalts and olivine-phyric basalts were collected by dredging and ALVIN sampling. The young volcanic centers were found along the lower pa rts of the south and north walls of the transform adjacent to and overlying much ol der highly-sedimented terrain of talus, pelagic sediment, and older manganese encruste d basalt. There is no indication of recent faulting within the young volcanics. The B-C fa ult is much shorter than the A-B fault (35 km vs. 18 km) and has less relief. N-MORB samples were primarily recovered from

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14 within this transform. The C-D fault and th e fault linking trough D and the ERTI are less well-defined and the linearity of the fa ults are not continuous to the SEPR. Samples used for this study were collected in 1991 aboard the Atlantis II cruise 125-25 and include rock dredges, rock cores, Alvin submersible dives (Figures 2-2 and 23). Eleven SeaBeam surveys were also c onducted during the 1991 crui se to add to prior SeaBeam and Sea MARC II survey data. The SeaBeam data allows identification of morphological features that have 10 m or mo re relief. Seventeen Alvin dives were completed within the Siqueiros transform domain and 171 samp les were collected (Table 2-1). Sample localities and geological re lationships are based on the ALVIN dive observations and SeaBeam survey data. Al vin dive tracks are based on the ALNAV network and SeaBeam maps with an estimat ed uncertainty of 100-200 m. Thirty-nine dredges and five rock cores were also c onducted in the Siqueiros transform domain (Table 2-2). The dredges consisted of a 50 cm x 1 m mouth frame 2 m chain bag with fishnet liner, and chain harness, with a 12,000 lb weaklink system. A cylindrical, lead depressor weight was used 100 m up the wire from the dredge mouth. Poor performance of onboard pingers forced the wire to be between 300-400 m greater than bottom depth to insure contact with the bottom. Dredge tr acks were kept short (<1 km) to maximize confidence in the sample localities and were located using Global Positioning System (GPS) and by correlating real-time Seabeam depths to existing maps along the sample track. The Siqueiros volcanic terrain mainly c onsists of pillow flows found within and around the intra-transform spreading centers a nd at small eruptive centers in transform shear zones. A few sheet flows have been found within the spreading basins. In contrast,

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15 Table 2-1. Siqueiros transf orm Alvin dive locations. Dive General Location of Dive # of Samples General Sample Descriptions 2375 2nd abyssal hill west of spreading center B axis 9 Fresh pillow basalts, one sediment sample 2376 Southern portion of spreading center B axis 11 Fresh to slightly weathered pillow basalts, one ropy lava 2377 Northern portion of spreading center B axis 11 Fresh basalts 2378 Southern crescent ridge of C and central graben 11 Fresh pillow and sheet basalts 2379 North wall of A-B fault just west of spreading center B intersection 3 Sediment covered microgabbros 2380 Southern RTI hole at spreading center B and trough east of spreading center B axis 12 Older pillow, lobate, and sheet basalts 2381 Southern wall of B-C fault 13 Older, somewhat weathered basalts 2382 Southern wall west of spreading center B and plateau south of transform 11 Fresh older sediment covered basalts. Some lobates and sheets. 2383 Southern ridge of spreading center A 8 Very fresh pillow basalts to slightly weathered basalts 2384 Young cones in axis of A-B fau lt 14 Very fresh, glassy basalts to older basalt fragments 2385 Northern RTI hole and central rift of spreading center C axis 9 Sheet, lobate, and pillow basalts, fresh to slightly weathered 2386 Trough and northern peak of D 8 Somewhat young hackly lava, mostly Mn coated older pillow and lobate basalts 2387 Cones in axis of B-C fault near intersection with spreading center C 9 Fresh-slightly altered basalts, mostly pillows 2388 A-B fault, cone on south side of axis and traverse up the north wall 14 Older sediment covered basalts and microgabbros 2389 Northern ridge in spreading center A 8 Fresh pillow lavas 2390 WRTI, small ridge that connects EPR to south wall of the A-B fault 9 Fresh – sediment covered pillow and lobate basalts 2391 Small cone built against south wall of the A-B fault west of southern spreading center A ridge 11 Sediment covered basalt, microgabbros

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16 Table 2-2. Siqueiros tran sform dredge locations. Dredge/Rock Core General Location of Dredge/Rock Core # of Samples A25-D1 Southern p ortion of s p readin g center B48 A25-D2 B-C Fault24 A25-D4 West of s p readin g center B12 A25-D5 West of s p readin g center B5 A25-D6 West of s p readin g center B2 A25-D7 West of s p readin g center B3 D25-D8 West of s p readin g center B1 A25-D9 West of s p readin g center BNo Recover y A25-D10 West of s p readin g center BNo Recover y A25-D12 South of s p readin g center BNo Recover y A25-D13 South of s p readin g center B1 A25-D14 South of s p readin g center B6 A25-D15 South of s p readin g center B1 A25-D16 Northwest of A-B faultNo Recover y A25-D17 Northwest of A-B fault10 A25-D18 Small rid g e p arallel hill west of s p readin g center B 6 A25-D19 South end of s p readin g center B10 A25-D20 Small cones near the mid p oint of A-B fault12 A25-D22 A-B fault5 A25-D23 A-B fault2 A25-D24 A-B fault3 A25-D25 Southwest side of s p readin g center C7 A25-D26 North of s p readin g center C10 A25-D27 West of s p readin g center C5 A25-D28 Eastern Rid g e Transform Intersection5 A25-D29 Eastern Rid g e Transform Intersection1 A25-D30 Eastern Rid g e Transform Intersection8 A25-D31 Eastern Rid g e Transform Intersection1 A25-D32 S p readin g center C6 A25-D33 S p readin g center C5 A25-D34 S p readin g center C5 A25-D35 Southwest of s p readin g center A7 A25-D36 East of s p readin g center A9 A25-D37 East of s p readin g center A3 A25-D38 East of s p readin g center A5 A25-D39 EPR ab y ssal hills4 A25-D43 Eastern Rid g e Transform Intersection5 A25-D44 Eastern Rid g e Transform Intersection2 RC-3 North of s p readin g center BLittle RC-11 South of s p readin g center BLittle RC-40 Eastern Rid g e Transform Intersection1 RC-41 Eastern Rid g e TransformIntersection1 RC-42 Eastern Rid g e Transform Intersection1

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17 the EPR north of the transform consists mainly of sheet or lobate flows emanating from the axis and occasional pillow flows, which ar e found near ridge tips and off axis (Perfit and Chadwick, 1998). Pillow flows are charac teristic of low effu sion rates suggesting that the intra-transform spreading centers are not as magmatically active as the adjacent EPR segments. The northern segment of the EPR extending up to the Clipperton transform is very well sampled. Little sampling has been done on the southern limb of the EPR.

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18 $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U %U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U $ T & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V &V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V W W W W W W W W W W W W W W W W W W W W W 2800 2700 2600 3200 3200 2900 3700 2600 3300 3800 2600 3100 3500 3700 2700 3200 2900 3100 3200 3000 2900 3400 3100 31 0 3200 3000 2900 3000 3100 N 100 m Contours# STrough D Ridge Transform Intersection' W $ TSpreading Center A% UA-B Fault& VSpreading Center B# SB-C Fault$ TSpreading Center C% UC-D Fault 815' 815' 830' 830' 10400' 10400' 10345' 10345' 10330' 10330' Figure 2-2. Bathymetry and sample locations for west side of the Siqueiros transform. 0 5 10 15 20 25 km

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19 % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U % U & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V &V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V & V #S #S #S #S #S#S #S #S #S #S #S #S #S #S #S#S#S#S & V & V & V & V & V & V & V & V & V & V & V & V $ T$ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T $ T % U % U $ T $ T $ T $ T $ T # S # S # S # S # S # S # S # S 3300 3700 2700 3200 0 00 3500 3200 3100 2200 3200 2300 2600 2400 2500 2700 3200 3000 2900 2900 3000 3400 3000 3200 3300 3100 2700 2800 2300 815' 815' 830' 830' 10330' 10330' 10315' 10315' 10300' 10300' N 100 m Contours# STrough D Ridge Transform Intersection' W $ TSpreading Center A% UA-B Fault& VSpreading Center B# SB-C Fault$ TSpreading Center C% UC-D Fault Figure 2-3. Bathymetry and sample locations for east side of the Siqueiros transform. 0 5 10 15 20 25 km

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20 CHAPTER 3 ANALYTICAL METHODS All Alvin samples were petrographically described and cataloged on ship. Representative dredge samples were inspect ed and slabbed with a rock saw for thinsection chips and in order to remove surface alteration. Glass rinds were removed from samples with glass and separated for furthe r cleaning. The glass and some whole rocks were crushed in a hardened steel mortar and then cleaned in acetone, 2N HCl, and distilled water in a heated ultrasonic bat h. The samples were then inspected under a binocular microscope and a ny alteration, sediment, or Mn -encrusted glasses were removed. A few samples that were heavily Mn-encrusted could not be completely cleaned and were labeled as “dirty” samples. After cleaning, 7-10 grams of glass or rock chips were crushed and powdered. The rema inder of the clean samples was saved for other analyses. Over 150 samp les were processed at sea. Major element analysis of the Siqueiros natural glass samples was done by electron microprobe at the US Geological Survey (USGS) in Denver using an ARLSEMQ microprobe and JEOL microprobe. An additional data set for major and minor elements was produced for the Siqueiros sa mples by analysis on a Cameca SX50 electron microprobe at the University of Tasmania (Danyushevsky, personal comm.). For the electron microprobe analysis, sub-samples of the cleaned glass chips were inspected using a binocular microscope and selected for analysis. All probe analyses were normalized to standard glasses VG-A99 and Jd F-D2 which were run concurrently with

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21 the Siqueiros glasses. Analyses were correct ed using the procedure of Meeker and Quick (1991). Whole-rock samples (glass plus phe nocrysts) were also analyzed by microprobe at the USGS after fusion in a rhenium strip furnace. Most of the Siqueiros samples were anal yzed for trace element contents (Co, Cu, Ga, Nb, Ni, Rb, Sr, Y, Zn, Zr, V, Cr, Ba, Sc, K, and Ti) by x-ray fluorescence spectrometery (XRF) in the department of Geological Sciences at the University of Florida using an automated ARL-8420+ spectrome ter. Approximately five grams of the powder samples were mixed with an organic binder and pressed into pellets for XRF analysis. Matrix absorption effects were acc ounted for based on the intensity of the Rh Compton peak (Reynolds, 1963). Standards were run no less frequently than every seven samples in order to correct for any fluctuat ions in the x-ray intensity or instrument conditions. Replicate analyses of rock sta ndards show that accuracy and precision are generally better than 2% for the elements C u, Zn, V, Ti, Sr, Y, and Zr, better than 5% for K, Rb, Nb, Ba, Co, Ga, and Ni, and to with in 10 % for Sc. Anal ytical precision is significantly worse (> 20%) for Nb, Rb, and Ba when abundances are near detection limits (3, 2, 10 ppm respectively). Direct cu rrent plasma (DCP) spectrometry was also used for phenocryst free glass separates at the Lamont-Doherty Earth Observatory. The DCP analysis included major and some minor and trace elements (Ba, Cr, Cu, Ni, Sc, Sr, V, Y, Zr, Mn, and Ti). A few samples were measured for trace elements (Y and Sc) and the rare earth elements (REEs) by inductiv ely coupled plasmamass spectrometry (ICPMS) at the University of Houston by Dr. J ohn Casey. Major element and trace element data was also analyzed by the Canadian Geological Survey by ICP-ASE. Chemical

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22 analyses of the Siqueiros picritic basalts were done by microprobe after fusion in a rhenium strip furnace. Duplicate analyses of selected samples we re completed by the different labs and by different methods. Because this has the potential to lead to systematic bias in the data, all of the data were graphically compared (Fig ure 3-1). The microprobe data from the ARLSEMQ microprobe and the JEOL microprobe show no apparent analytical offsets. Nor are there any significant differences between the microprobe data and the DCP data. However, the MgO, and P2O5, data from the Cameca SX50 elect ron microprobe appear to have a slight offsets when compared to the other Siqueiros electron microprobe data. The Cameca SX50 electron microprobe MgO and P2O5 data was normalized to match the other electron microprobe MgO and P2O5 data (Figure 3-2). The normalization method is discussed in Appendix A. The Cr2O3 contents obtained on the Cameca SX50 electron microprobe are not directly comparable to mo st of the other Cr data (mostly XRF). Measurements are made on the glass composition whereas the XRF analyses are of the whole rock samples. Therefore, it was not possible to use the microprobe Cr2O3 contents to compare with the Cr2O3 contents of the other Siqueiros samples. Since the XRF is a much more accurate method for obtaining Cr contents, only the Cr2O3 contents determined by XRF were used in this study.

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23 23 0.5 1 1.5 2 2.5 3TiO2 2 2.2 2.4 2.6 2.8 3 3.2Na2O 47 48 49 50 51 52 56789101112SiO2MgO 9 9.5 10 10.5 11 11.5 12 12.5 13 56789101112CaOMgO ARL-SEMQ Microprobe Samples JEOL Microprobe Samples Cameca SX50 Samples DCP Samples Figure 3-1. Graphical comparison of ARL microprobe, JEOL microprob e, DCP, and Cameca SX50 data before correction of the data.

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24 24 6 7 8 9 10 11 12 13 5678910111213FeOMgO 0.1 0.2 0.3 0.4 0.5P2O5 12 13 14 15 16 17 18 19Al2O3 56789101112 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7K2OMgO ARL-SEMQ Microprobe Samples JEOL Microprobe Samples Cameca SX50 Samples DCP Samples Figure 3-1. Continued.

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25 25 1 1.5 2 2.5 3TiO2 2.2 2.4 2.6 2.8 3 3.2Na2O 47 48 49 50 51 56789101112SiO2MgO 56789101112 9 10 11 12 13 14CaOMgO ARL-SEMQ Microprobe Samples JEOL Microprobe Samples Cameca SX50 Samples DCP Samples Figure 3-2. Comparison of data after ad justment of the Cameca SX50 MgO and P2O5 contents.

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26 26 7 8 9 10 11 12 13FeO 0.1 0.2 0.3 0.4 0.5 0.6P2O5 12 13 14 15 16 17 18 56789101112Al2O3MgO 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 56789101112K2OMgO ARL-SEMQ Microprobe Samples JEOL Microprobe Samples Cameca SX50 Samples DCP Samples Figure 3-2. Continued.

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27 The chemical characteristics of the samples were determined by comparing their major and trace element abundances to each ot her and samples from the adjacent East Pacific Rise (EPR) and from the Garrett tran sform. Liquid lines of descent (LLDs) and rare-earth element diagrams were used to help group samples of similar parental compositions. SeaBeam and SeaMarcII sonar data has also been collected for the Siqueiros transform (Fornari et al., 1989) The sonar data along with the ALVIN submersible dive observations were used to determine sample locations with respect to the local geologic/structural features. Th e ALVIN submersible observations and dive track data were used to precisely locate sa mples and to create de pth profiles along dive transects. GIS (Arcview) data files were created that consist of latitude/longitude, elevation (depth), geologic location, and chem ical characteristics. The data files were then used to create geologic maps of the Siqueiros transform. Thin-sections of 60 samples were studie d with a petrographic microscope to identify the different phases in each sa mple and to provide information regarding crystallization and mixing histories during pe trogenesis. Microprobe analyses of spinel, olivine, and plagioclase phenocrysts were also completed for many of the samples. The compositions of the phenocrysts were then us ed to better understa nd crystallization and mixing histories.

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28 CHAPTER 4 PETROGRAPHY AND LOCAL GEOLOGIC RELATIONSHIPS The rocks from the Siqueiros transform include picrites, picritic basalts, basalts, and a few microgabbros. Thin sections examined in this study were cut from the outer glassy rinds as well as the more crystalline interiors of 63 samples. The majority of the samples chosen for thin section analysis we re recovered from the A-B fault, but thin sections were made from samples from all sp reading centers and fau lts and of one sample from the RTI. Descriptions of the thin sect ions examined are provided in Table 4-1 and representative photomicrographs are shown in Figures 4-1, 4-2, and 4-3. A few of the samples are aphyric or vitrophyric, contai ning less than 1 volum e % phenocrysts and microphenocrysts, but most of the sample s are phyric containing greater than 5% microphenocrysts. Samples from the A-B Fault The majority of the thin sections are fr om the A-B fault because these samples are unusually olivine-rich with 5-20 modal% oliv ine phenocrysts (Perfit et al., 1996). The samples are remarkably fresh and unaltered, w ith thick glassy rinds which are free of palagonitization and Mn-coatings; indications of the relative youthfulness of the lava (Perfit et al., 1996). The samples differ from th e rest of the Siqueiros samples in that they contain only olivine and spinel phenocrysts. Abu ndant olivine microphenocrysts are found in the glass and at centers of variolites. Near the inte riors there is minor dendritic plagioclase microphenocrysts radiating from oliv ines. Despite the gr eat recovery depths (3000-3900m), many of the olivine-rich basalts from the A-B fault have a greater degree

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29Table 4-1. Thin section descriptions. phenocryst texture microphenocryst texture Sample # Loc MgO wt% glass Ve olivine plag spinel olivine plag cpx remarks 2389-3 A ND Y Y Rounded, embayed Embayed, zonedNone Hopper euhedral Swallow tail tabular None Porphyritic; variolitic; flow features around phenocrysts; few plag clots 2389-8 A ND Y Y Very few, rounded Slight zoning, embayed, skeletal None Subhedral Swallow tail, acicular None Porphyritic; plag. clots; variolitic intersertial 2389-8A A ND Y Y Rounded, skeletal Huge, embayed, skeletal, zoning None Subhedral Acicular tabular None Porphyritic; huge plag. clots up to 9 mm; variolitic 2389-1P A 7.35 Y Few Rounded Oscillatory zoning, skeletal, rounded None Variolitic Variolitic None Porphyritic; variolites around ol microphenocrysts 2384-1 A-B 9.6 Y Y Up to 6 mm, rounded, skeletal, embayed None Skeletal, rounded, inside & outside olivine Hopper dendritic Swallow tail None Porphyritic; primarily microphenocrysts; intersertial with opaques 2384-10 A-B 9.59 Y Few 1-4 mm, skeletal None Inside & outside olivine Dendritic Swallow tail None Porphyritic; variolitic dendritic; large ol. clots 2384-11 A-B 8.79 Y Very few Few, embayed None Few inside olivine microphenocrysts Hoppersubhedral Swallow tail None Porphyritic; varioliticintersertial; primarily quenched glass with plag. & ol. microphenocrysts 2384-12 A-B 9.11 Y Very few Skeletal, rounded Very few, skeletal Rounded; inside & outside olivine Dendritic Swallow tail tabular None Porphyritic; ol. & plag. clots; intersertial with opaques 2384-13 A-B 8.53 Y Y 1-2 mm, skeletal None Inside olivines Dendritic Swallow tail None Green alteration in vesicals; variolitic-dendritic with opaques; dendritic growth around ol. 2384-3 A-B 10.1 Y Few 6-7 mm, rounded, skeletal None Skeletal, zoned edges Hopper Swallow tail dendritic None Porphyritic; varioliticdendritic with opaques; primarily microlites

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30Table 4-1. Continued. phenocryst texture microphenocryst texture Sample # Loc MgO wt% glass Ve olivine plag spinel olivine plag cpx remarks 2384-4B A-B ND Y Y Skeletal, embayed Zoned, embayed, skeletal None Hopper Tabularswallow tail Possibly quenched in g.m. Porphyritic; 6-7 mm clots of ol. & plag.; intersertial g.m. with opaques; alteration inside vesicals 2384-4C A-B ND Y Y Rounded, skeletal Skeletal, zoned, sieve texture None Hopper Swallow tail Possibly quenched in g.m. Porphyritic; intersertial with opaques; primarily plag. phenocrysts vs. ol. 2384-6 A-B 9.57 Y Few Many, 4-5 mm; skeletal, resorbed None Skeletal Hopper Tabularswallow tail None Porphyritic; variolitic; euhedral spinel microphenocrysts; 2 populations of plag. microphenocrysts 2384-7 A-B 9.9 Y None Skeletal None Resorbed Yes Swallow tail None Porphyritic; sparsely phyric; variolitic with ol. and plag. microphenocrysts 2384-9 A-B 9.73 Y None 5-6 mm, embayed, rounded, skeletal None Skeletal, variolites around Variolitic around None None Almost all glass with variolites and ol. microphenocrsts and phenocrysts D20-6 A-B 9.87 Y Few 2-3 mm, rounded, embayed, skeletal None Inside and next to olivine Dendritic Dendritic None Porphyritic; primarily glass with microphenocrysts; rounded spinel microphenocrysts; varioliticdendritic with opaques D20-5 A-B 10.6 Y None 2-3 mm, skeletal None Skeletal, zoned Hoppper Dendritic None Porphyritic; glass-varioliticdendritic 2379-2 A-B 6.91 (WR) Y Very few Rounded Skeletal None Tabular Tabular None Poikilitic; intersertial with large opaques; ol. and plag. intergrown 2379-2 A-B 6.91 (WR) Y Few 4-6 mm, embayed Sieve texture None Tabular Tabular None Poikilitic; intersertial g.m. with large opaques; ol. and plag. intergrown

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31Table 4-1. Continued. phenocryst texture microphenocryst texture Sample # Loc MgO wt% glass Ve olivine plag spinel olivine plag cpx remarks 2384-1 A-B 9.6N Y Rounded, skeletal None Inside and outside olivine, skeletal, embayed Tabular Swallow tail Possibly quenched in g.m. Porphyritic; intergranular with opaques 2384-10 A-B 9.59Y Y Few, about 1 mm, roundedNone Skeletal, zoned Dendritic Dendritic swallow tail None Primarily microlitic; varioliticdendritic 2388-6 A-B ND None Few Rounded, altered edges Zoning, skeletal, embayed None Few, rounded Tabular Possibly quenched in g.m. Porphyritic; intergranular with opaques; large plag. & ol. clots 3-4 mm 23886WR A-B ND None Few Embayedsubhedral, huge Huge, resorbed edges, skeletal, with very slight zoning Inside olivines Rare, rounded Swallow tail tabular Possibly quenched in g.m. Porphyritic; intergranular; microphenocrysts align around plag. Phenocrysts; plag & ol clots about 5 mm 2388-7 A-B ND None Few None Few, about 4 mm, embayed, skeletal, resorbed rims None Rare, broken up, embayed Subhedral Possibly quenched in g.m. Primarily microlitic; intergranular with opaques 2391-10 A-B ND None Y None Embayed None Hopper Tabular anhedral Possibly quenched in g.m. Poikilitic; intersertial with opaques 2391-10 A-B ND None Y None Embayed None Subhedralanhedral Tabular Possibly quenched in g.m. Poikilitic; intersertial with opaques 2391-6 A-B ND Y Y Very few, skeletal Embayed, skeletal, zoning,sieve texture None Intergranular with plagioclase Tabular swallow tail Possibly quenched in g.m. Porphyritic; intersertial with opaques; clots of plag. phenocrysts 2391-7 A-B ND Y Y Skeletal, rounded Skeletal, embayed, zoning None Hopper subhedral Acicular swallow tail Possibly quenched in g.m. Porphyritic; intersertial with opaques; plag. clots 3-4 mm

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32Table 4-1. Continued. phenocryst texture microphenocryst texture Sample # Loc MgO wt% glass Ve olivine plag spinel olivine plag cpx remarks A25D20-8 A-B ND Y None Embayed, skeletal, rounded None Rounded, embayed, slightly zoned edges Hopperdendritic Very few, dendritic None Phorphyritic; mainly glass; variolitic with hopper ol. microphenocrysts and very few dendritic plag. microphenocrysts D-17B A-B ND Y Y Rounded, skeletal Skeletal skeletal, zoned edges Few, dendritic Dendritic None Phorphyritic; ol. & plag. clots; dendritic D17-4 A-B ND None few Intergrown with plag Skeletal, embayed None Subhedral Subhedral Possibly quenched in g.m. Poikilitic; intersertial with opaques; green alteration D20-5 A-B 10.6Y few Skeletal, rounded None Lots, zoned rims, skeletal Hopper Very few; dendritic None Sparsely porphyritic; mainly glass; variolitic dendritic with spinel microphenocrysts D20-6 A-B 9.87Y Y Skeletal, rounded None Zoned rims, skeletal Hopper Swallow tail dendritic Possibly quenched in g.m. Porphyritic; variolitic dendritic with opaques; mainly hopper ol. in glass 2384-11 A-B 8.79Y Very few Few, about 1 mm, skeletal, embayed None None Subhedral Swallow tail dendritic None Primarily microlitic; varioliticdendritic 2384-11 A-B 8.79Y Few Few, about 1 mm, skeletal, embayed None None Subhedral Swallow tail dendritic None Primarily microlitic; varioliticdendritic; aligned plag. microphenocrysts 2384-12 A-B 9.11Y Y Few, skeletalNone Few, around olivines Intergown with plagioclase Swallow tail Possibly quenched in g.m. Primarily microlitic; intergranular with opaques 2384-13 A-B 8.53Y Y Skeletal None None Slightly hopper Swallow tail dendritic None Porphyritic; variolitic dendritic; ol. clots with dendritic growth of plag around

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33Table 4-1. Continued. phenocryst texture microphenocryst texture Sample # Loc MgO wt% glass Ve olivine plag spinel olivine plag cpx remarks 2384-14 A-B 7.23None Y 5-6 mm, skeletal, rounded Skeletal, slight zoning None Subhedral Swallow tail Possibly quenched in g.m. Porphyritic; intersertial with opaques clots of ol. and plag. phenocrysts 2384-14 A-B 7.23None y Skeletal Skeletal None Subhedral Tabular Possibly quenched in g.m. Porphyritic; intersertial with opaques 2384-2 A-B 9.54Y Skeletal, 5-6 mm None Rounded, zoned rims Hopperdendritic Swallow tail tabular None Porphyritic; intersertial with fine opaques 2384-4 A-B ND Y Y Rounded, embayed, in clot Embayed, skeletal, zoning in some Inside olivines Slightly hopper Swallow tail Possibly quenched in g.m. Porphyritic; intersertial with opaques; circular alteration; alignment around clots 2384-4A A-B 8.35Y Y Skeletal, embayed Skeletal, zoningInside olivines Subhedralhopper Swallow tail None Porphyritic; intersertial with opaques; huge clots of plag. & ol. 2384-4C A-B ND Y Y Skeletal Embayed, zoning Inside olivines Subhedralhopper Swallow tail tabular None Porphyritic; intersertial g.m. with opaques; clots of ol & plag 2-3 mm 2388-1 A-B ND N Many None Very few, skeletal None Few, skeletal, rounded Tabular Possibly quenched in g.m. Poikilitic; opaques in g.m. 2391-9 A-B ND N Few, irregular None Very resorbed None Subhedral -anhedral Tabular Possibly quenched in g.m. Poikilitic 2391-8 A-B ND N Y, alteration around Resorbed, skeletal, in clots Skeletal, resorbed, zoned A lteration around, rounded Few, subhedral Acicular Possibly quenched in g.m. Porphyritic; intergranular with opaques; primarily plag. in clots 2-3 mm 2391-2 A-B 7.84N Very few Skeletal, embayed Skeletal, embayed, zoning None Few, hoppersubhedral Acicular Possibly quenched in g.m. Phorphyritic; green-brown alteration; plag. clots

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34 Table 4-1. Continued. phenocryst texture microphenocryst texture Sample # Loc MgO wt% glass Ve olivine plag spinel olivine plag cpx remarks 2376-8 B 8.02Y Y Rounded, skeletal Skeletal, embayed, sieve texture None Hopperdendritic Swallow tail tabular None Porphyritic; variolitic intersertial with opaques; 45 mm clots of plag. & ol. 2376-9 B ND Y Y Skeletal, embayed Zoned, skeletal Zoned rim Dendritic Dendritic None Porphyritic; 3 mm clots of ol. & plag.; variolitic dendritic with opaques; 3 populations of plag 2382-10 B 6.68 (WR) Y None Skeletal Sieve texture, rounded, zoned, skeletal None Tabular Swallow tail Possibly quenched in g.m. Porphyritic; intersertial with opaques; mainly plag. clots with minor ol. 2376-11 B ND N Embayed, rounded Skeletal, oscillatory zoning, huge None Subhedral Tabular None Porphyritic; variolitic intersertial with opaques; plag. & ol. clots about 4 mm 2387-1 B-C ND Y Few Rounded Sieve texture, zoning, skeletal None Tabular Swallow tail None Phorphyritic; intersertial with opaques 2387-2 B-C 7.56None Y, glass filled Out of equilibrium, not as big as plag. Oscillatory zoning, sieve texture, skeletal None Small, subhedral Swallow tail tabular None Porphyritic; 2-3 mm clots of large plag. with small ol. & plag.; variolitic intersertial; brown and green alteration 2387-2 B-C 7.56Y Y Few Embayed, zoning, skeletal, sieve texture None Subhedral euhedral, skeletal Swallow tail tabular None Porphyritic; 2-3 mm clots of large plag. with small ol. & plag.; variolitic intersertial; brown and green alteration 2387-5 B-C 7.79None Y Few, embayed, skeletal Zoning, embayed, sieve texture None Subhedral Tabular Possibly quenched in g.m. Porphyritic; 5-6 mm clots of plag. & ol. phenocrysts and microphenocrysts; intersertial with opaques 2387-7 B-C ND None Y Very rare, rounded Zoning, embayed, sieve texture None Anhedral Anhedral Possibly quenched in g.m. Poikilitic; 5 mm plag. clots; intergranular with opaques

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35Table 4-1. Continued. phenocryst texture microphenocryst texture Sample # Loc MgO wt% glass Ve olivine plag spinel olivine plag cpx remarks 2385-1 C ND Y Few Embayed, very few Rounded, skeletal, sieve texture Zoned, skeletal Hopper tabular Swallow tail None Porphyritic; varioliticintersertial 2385-1 C ND Y Few Embayed, very few Rounded, skeletal, sieve texture Zoned, skeletal Hopper tabular Swallow tail None Porphyritic; varioliticintersertial D34-2 C-D 9.12 Y Y Few, skeletal Few, skeletal None Skeletal Acicular tabular Possibly quenched in g.m. Microlitic; intersertial with opaques 2386-3 D ND N Y, many None One about 1 mm, embayed None Anhedral subhedral Subhedral Possibly quenched in g.m. Primarily microlitic; few plag. clots; intersertial with opaques 2386-6 D ND N Y, many None Few, small embayed None Anhedral subhedral Swallow tail tabular Possibly quenched in g.m. Primarily microlitic; few plag. clots 2386-8 D ND None Y None Clots None Anhedral subhedral Tabular Possibly quenched in g.m. Porphyritic; plag. clots; intersertial with opaques 2386-1 D ND Y None One about 1 mm, embayed None Subhedral Subhedral Possibly quenched in g.m. Microlitic; intergranular with ol., plag., and opaque microphenocrysts 2390-6 WRTI ND Y Y Skeletal Skeletal, embayed None Hopper Acicular dendritic None Primarily microlitic; intersertial with opaques; primarily plag. Notes: ND = no data; Ve = vesicals.

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36 A. B. C. D. Figure 4-1. Photomicrographs taken under pl ain light (a & b) and cross polarized light (c & d). Picture width is equal to approximately 1.8 mm. A) Rounded and embayed olivine phe nocrysts surrounded by glass and variolites (sample 2384-7). B) Olivine microphenocrysts in transition from variolitic to dentritic texture (sample 2384-7). C) Plagioclase microphenocrysts growing around olivine glomerophenocryst in sample 2384-13 a picritic ba salt. D) Plagioclase and olivine phenocrysts in interserti al groundmass with opaques and quenc hed clinopyroxene (sample 2391-10).

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37A. B. C. D. Figure 4-2. Photomicrographs take n under plain light (a, b, & d) and cross polarized light (c). Picture width is equal to app roximately 1.8 mm. A) Edge of large plagioclase with very resorbed edge su rrounded by olivine, plagioclase and opaque microphenocrysts (sample 2387-7). B) Rounded olivine phenocrysts in dendr itic groundmass with hopper olivine microphenocrysts (sample D20-5). C) Pl agioclase phenocrysts with oscillatory zoning in va riolitic groundmass with olivine and plagioclase microphenocrysts (s ample 2389-1). D) Rounded olivine phenocryst s with spinel inside attached in variolitic groundmass (sample 2384-3).

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38A. B. C. D. Figure 4-3. Photomicrographs ta ken under plain light (c) and cross polarized light (a, b, & d) Picture width is equal to app roximately 1.8 mm. A) Plagioclase and olivine clot in intersertial groundmass of plagio clase and olivine microphenocrysts (sample 2384-4). B) Rounded olivine phenocrysts in intersertial grou ndmass of olivine and sligh tly swallow tail plagioclase microphenocrysts (sample 2384-1). C) Ci rcular variolites with olivine microphenocrysts (sample 2384-9). D) Small olivine plagioclase clot within intersertial groundmass of plagioclase, o livine, and quenched clinopyroxene (sample 23844).

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39 of vesicularity than EPR basalts usuall y found between 2500m to 2800m depth. Several dredges were targeted to sample the cone-sha ped features in the axis of the A-B fault. The position of these features, roughly halfwa y along the fault, suggested they would be located in older volcanic terr ane; but their morphology s uggested recent constructional volcanism. Dredge D-20 started in an area of small cones near the midpoint of the A-B fault and proceeded up the south wall of the fault valley. La rge quantities of younglooking glassy basalts, many of which were pi critic were recovered in the dredge. Three additional dredges (D-22, D-23, and D-24) were carried out in the deep areas of the A-B fault, on small, closed-contour peaks and sa ddles in the axis of the trough, and up the middle to lower walls of the fault valley (Fig ure 4-4). Young, glassy pillow basalts were found throughout the A-B fault. To more comp letely document the loci of eruption and tectonic setting of the young-l ooking, olivine-rich basalts Al vin Dive 2384 traversed the floor of the A-B fault and went up the south wa ll across on the cone-like features (Figures 4-5). The basalt-floored depression where dive 2384 bega n was found to be nearly devoid of sediment which is unusual for tran sform faults. Fresh olivine-phyric basalts were recovered from a volcanic slope characterized by intermingled pillows, broken pillows, and fragments of basalt all fresh and glassy. White to yellow staining, inferred to be hydrothermal in origin, was abundant on the broken basalt surfaces. Another field of fresh basalt flows was found along the base of the south wall of the fault valley. Here nearly intact pillows and tubes were found. Th e north side of the fault axis consisted of rugged constructional volcanic terrane with occasional steep -sided (up to 70), flattopped, volcanic “haystacks”. Glassy picritic ba salts with little or no Mn coatings were recovered from a free-standing cone. Around the cone, the seafloor was built of glassy

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40 Figure 4-4. Dredge and Alvin dive locations wi thin the A-B fault.

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41 Figure 4-5. Alvin di ve 2384 traverse.

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42 basalt flows and veneered with basalt bl ocks and glass shards. To the north, a bathymetric notch separates the younger volcanic terrane of the south from an older terrane to the north (Figure 45). Here sediment was more abundant and glass is absent. Only plagioclase-phyric or olivine + plagiocl ase-phyric basalts were recovered from the older lava flows. Within the 28 km long A-B fault, 9 cone like features believed to be constructional volcanic features have been identified (Wendlandt and Ridley, 1994). Bulbous to elongate pillows are the dominant basalt morphol ogy within the A-B fault, similar to that observed along intra-transform spreading cente rs, but in marked contrast to the lava morphology at the EPR axis, wher e sheet flows and lobate form s dominate (Ballard et al., 1981; Kastens et al., 1986; Perfit et al., 1991). The erupted lavas overflow a severely tectonized terrane on the valle y walls, but the young flows fr om which the olivine-rich samples were recovered had little structural disruption of the flow surfaces. The olivine rich basalts recovered from the A-B fault valle y are inferred to have erupted recently as evidenced by the extreme freshness of the gla ssy lava surfaces, th in to non-existent sediment cover, and relatively minor struct ural disruption of the flow surface. The youngest-looking basalts are as glassy as young lavas that floor the axial summit caldera on the EPR between 930’-54’ N (Haymon et al., 1991, 1993; Fornari et al., 1991; Perfit et al., 1991). Siqueiros Sample Petrography The extremely olivine-rich picrites and picritic sample s which lack plagioclase phenocrysts were only recovered within the AB fault. The older looking, Mn-encrusted basalt samples from the talus and sediment -covered terrains surr ounding the fresh flows are plagioclase + olivine spin el-phyric, or plagioclase sp inel-phyric (Perfit et al.,

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43 1996). The samples from the other faults, th e three spreading cente rs, tough D, and the RTIs are also plagioclase + olivine spinelphyric, or plagioclase spinel-phyric. Most samples are porphyritic, containing ph enocrysts and microphenocrysts. The olivine-rich samples tend to be hypohya line having glassy margins with sparse microphenocrysts and variolitic or dendritic in teriors. The plagioclase phyric samples range from hypohyaline to hypocrystalline or hol ocrystalline with nearly completely crystalline interiors and margins and only mi nor glass in the groundmass. Olivine and plagioclase microphenocrysts are present in al most all of the samples and a few samples had microphenocrysts of spinel. The more crystalline samples have opaques in the groundmass. Clinopyroxene phenocrysts have not been identified, but in the more crystalline samples clinopyroxene appears as a quenched phase in the groundmass. Olivine microphenocrysts are commonly found in centers of variolites or with dendritic growth of plagioclase surr ounding them. Olivines range from hopper crystals in the variolitic samples to subhedral in the more crystalline samples. Plagioclase microphenocrysts usually have swallow tail to dendritic forms. Euhedral and subhedral tabular plagioclase crystals are abundant in the more crystalline samples. In all the samples, including the olivine-rich samples from the A-B fault, the large phenocrysts have textures indicating that they were not in equilibrium with their host melt. Subhedral to euhedral olivine phenocrysts vary from 1 mm to 7 mm in size, but tend to have rounded or embayed edges and ar e often skeletal. Many of the samples are glomeroporphyritic with clots of olivine a nd plagioclase growing together. Larger plagioclase phenocrysts are commonly glomer oporphyritic and have skeletal, rounded or embayed edges. Many of the larger plagio clase phenocrysts also exhibit oscillatory

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44 zoning and some have sieve or moth-eaten text ures. Spinel phenocrysts exist inside and outside olivine phenocrysts. The spinels we re usually rounded, sometimes skeletal, and typically red to brown with darker rims. C linopyroxene is absent from all samples, which is unusual for MORBs. However, because most of the thin sections were made of the samples from the A-B fault, the samples ar e fairly primitive and clinopyroxene is not expected on or near the liquidus of more prim itive samples at low to moderate pressures. Crystal Liquid Equilibria In addition to thin sections, elemental analyses was completed by microprobe on olivine, plagioclase, and spinel crystals (A ppendix B). The olivine microprobe analysis was done on small, medium, and large olivin e phenocrysts. A comparison of the Mg# (Mg2+/ (Mg2+ + Fe2+)) of the glass surroundi ng the olivine and the Fo content of the olivine shows a strong correlation (Figure 46). The total compos itional range in the Siqueiros basalts is from Fo90.9 to Fo80.0, which is a slightly great er range than that found in olivine microphenocrysts and microlites within MORBs from the East Pacific Rise at 930’N. Olivine phenocrysts in the 930’N lavas have been found to range from Fo88 to Fo82 (Pan and Batiza, 2003). With successive fractional crystallization the Mg# of the melt and the Fo content of the crystallizing olivine both decrease. In the Siqueiros sample suite, there does not seem to be a co rrelation between the size and Fo content of the olivine. Although the majority of th e large olivines have more forsteritic compositions, some of the large olivines have lower Fo contents and some of the small olivines have relatively high Fo contents. Fo r the larger olivine phe nocrysts, successive microprobe analyses were done from the center outward to the rim. Individual crystals have little chemical zonation. Some olivin es show slight reverse zoning with the Fo content increasing from the core to rim (2384-3-ol1), while other samples are normally

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45 35 40 45 50 55 60 65 70 75 7075808590Mg # of GlassFo content of olivine Core of small olivine Interior of small olivine Rim of small olivine Core of medium olivine Interior of medium olivine Rim of medium olivine Core of large olivine Interior of large olivine Rim of large olivine 2377-7 D34-2 2384-9 2384-9 High Pressure Figure 4-6. Comparison of olivine fo rsterite content with the Mg# (Mg2+/(Mg2+ + Fe2+)) of the host glass. Modeled equilibriu m trends expected during fractional crystallization were calculated using the low pressure model of Danyushevsky (2001) for three different parental co mpositions. High pressure model of Danyushevsky (2001) also shown for comparison. zoned and the Fo content decreases slightly fr om the core to rim (2388-3a-ol1). Liquidmineral equilbria for olivine (Mg# versus th e Fo content) were calculated using the method of Danyushevsky, (2001) (Figure 4-6). Slightly different results were obtained using the ol-liquid equilibria model of Herz berg & O’Hara (2002). Fo-liquid equilbria were determined for three different parental li quids as they fractiona lly crystallized using the Petrolog program. There is almost no di fference for the three parental composition used in the major element models and ther e is no difference between the high and low

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46 pressure models. The Fo content of olivin e steadily decreases as the Mg# of the host magma decreases and as the oliv ine-melt partition coefficient (Dol-melt – Feol MgL/Mgol FeL) increases slightly with decreasing temp erature. The Herzburg & O’Hara (2002) model calculations predict sli ghtly lower olivine partition coefficients, which fit the observed data better (Figure 4-7). The oliv ine-melt partition coefficient in MORB lavas has been found to range from approxima tely 0.31 to 0.28 (Pan and Batiza, 2003). Olivine-liquid pairs in Siqueiros samples fall on the low side of this range of partition coefficients, requiring Kds of less than 0.28 if the olivine phenocry sts are actually in equilibrium with the host magma (Figure 48), alternatively, the olivines may be xenocrystic. The more Fo compositions of th e Siqueiros olivines suggests that many of the olivine phenocrysts came from more Mg-r ich melts and are out of equilibrium with the host magma. This is also suggested by the embayed edges and skeletal textures found in many of the olivine phenocrysts. Chemical analysis of plagioclase by microprobe shows that the plagioclase phenocrysts have much greater compositiona l variations than olivine phenocrysts (Appendix B). Analyses were completed on small and large plagioclase crystals and include core, rim, and interior zones. The total compositional range of plagioclase phenocrysts is from An58.0 to An88.3, compared to a composition range of An52.1 to An83.4 for phenocrysts from 93’N on the EPR (Pan & Batiza, 2003). The average Siqueiros plagioclase is slightly more calcic (An75.9) than the average plagioclase from 930’N on the EPR (An685). Many of the large plagioclase crys tals show visible zoning. Core to rim analysis show that the zones exhibit osci llatory zoning (Figure 49). Overall, the

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47 40 45 50 55 60 65 70 75 7075808590Mg# of GlassFo content of olivine Core of small olivine Interior of small olivine Rim of small olivine Core of medium olivine Interior of medium olivine Rim of medium olivine Core of large olivine Interior of large olivine Rim of large olivine 2377-7 D34-2 2384-9 2384-9 High Pressure Figure 4-7. Comparison of Olivine fo rsterite content with the Mg# (Mg2+/(Mg2+ + Fe2+)) of the host glass. Modeled equilibriu m trends expected during fractional crystallization were calculated using the low pressure model of Herzburg & O’Hara (2002) for three different parent al compositions. High pressure model of Danyushevsky (2001) also shown for comparison.

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48 40 45 50 55 60 65 70 75 80 7580859095Glass Mg#Fo content of olivine K (melt/ol) = 0.32 0.30 0.28 Figure 4-8. Calculated Fo contents of olivin e for partition coefficien ts ranging from 0.28 to 0.32. Siqueiros samples fall on the low side of the acceptable olivine-melt partition coefficients indicating that th ey are from more mafic magmas than that of the host glass.

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49 65 70 75 80 85 902377-11, plagioclase 11Plagioclase An content 62 64 66 68 70 72 742388-3a, plagioclase 7Plagioclase An content 81 82 83 84 85 862377-3, plagioclase 2Plagioclase An content 72 74 76 78 80 020406080100D15-1, plagioclase 1Plagioclase An contentCORERIM Figure 4-9. An contents for core, interior, and r im locations in Siqueiros plagioclase phenocrysts.

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50 50 55 60 65 70 75 80 85 90 4567891011Plagioclase An contentMgO Core of Small Plag Interior of Small Plag Rim of Small Plag Core of Large Plag Interior of Large Plag Rim of Large Plag 2377-7 D34-2 2384-9 2384-9 High Pressure Figure 4-10. Comparison of plagioclase An content from Siqueiros samples and An content evolution for thr ee of the major element parental compositions. Model equilibrium trends expected during fractional crystallization were calculated using low pressure model of Danyushevsky (2001). smaller plagioclase laths have lower An cont ents (Avg. An content = 69.9) than the larger laths (Avg. An content = 78.3). Anorthite cont ents of the small plagioclase phenocrysts exhibit a decrease with magma evolution (decr easing MgO) as predicte d in the fractional crystallization models (Figur e 4-10). Larger plagioclas e phenocrysts and a few of the smaller phenocrysts have higher An contents (for a given MgO or CaO) than predicted and have large variations in An content from core to rim (core-rim), suggesting that the phenocrysts are out of equilibrium with the host glass. The relationship between An content and the Ca# (Ca/(Ca + Na )) of the magma is complicated. Calculated plagioclase-liquid equilbria indicate that the Ca# initially

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51 decreases slowly with fractionation, but once clinopyroxene begins fr actionating from the liquid the Ca# decreases rapidly. Models that describe the relationship between the Ca# of the magma and the coexisting plagiocl ase An content were produced using the alogrithims of Danyushevsky, (2001) and Langmuir et al. (1992) (Fi g. 4-11 and 4-12). The best fit to the observed data were pr oduced using the plagioclase-liquid equilbria model of Danyushevsky, (2001). The compositi ons of most of the small plagioclase crystals follow the general trend predicte d for fractional crystallization, suggesting plagioclase compositions are controlled by the evolving magma chemistry. Large phenocrysts have higher An contents than predicted by the model (Figure 4-11). The higher An contents suggest that the phenocrysts Ca# content. Such high Ca# plagioclase phenocrysts have been found by others, but high Ca# melts are not commonly found in MORB (Ridley et al., in prep ; Pan and Batiza, 2003). Chemical analyses of cores of small spin el phenocrysts and cores, interiors, and rims of the larger spinel phenocrysts are presented in A ppendix B. The Cr# (100*Cr/(Cr + Al)) of the spinels range from 25-58 and gene rally decrease from core to rim (Figure 413). The Fe2O3 content (2.08-7.07 wt. %) and TiO2 content (0.12-0.98 wt. %) of the spinels are low; similar to other spinels from MORB (Dick and Bullen, 1984; Allan et al., 1988). The Fe3+/(Cr +Al + Fe3+) vs. Fe2+/(Mg + Fe2+) compositions of the Siqueiros spinels fall within the range and have trends observed in spinels from other MORB lavas (Figures 4-14). The Siqueiros spinels genera lly follow the Cr-Al trend observed in other spinel suites, but the larger phenocrysts in pa rticular, have low Fe2+/(Mg + Fe2+) for a

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52 505560657075808590 55 60 65 70 75 80Plagioclase An contentGlass Ca# Core of Small Plag Interior of Small Plag Rim of Small Plag Core of Large Plag Interior of Large Plag Rim of Large Plag 2377-7 D34-2 2384-9 2384-9 High Pressure Figure 4-11. Comparison of the host glass Ca# (100*C a/(Ca + Na) with the plagioclase An content. Modeled equilibrium trends expected during fractional crystallization were calculated using low pressure model of Danyushevsky (2001). Modeled trends fit the many of the smaller plagioclase crystals, but are unable to explain many of the samples that have high An contents compared with the Ca# of their host glass.

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53 505560657075808590 60 62 64 66 68 70 72 74 76Plagioclase An contentGlass Ca# Core of Small Plag Interior of Small Plag Rim of Small Plag Core of Large Plag Interior of Large Plag Rim of Large Plag 2377-7 D34-2 2384-9 2384-9 High Pressure Figure 4-12. Comparison of Siqueiros plagio clase An content vs. glass Ca# (100*Ca/(Ca +Na)). Modeled equilibrium trends expected during fractional crystallization were calculated using the low pressure model of Langmuir et al. (1992). High pressure model is also shown for comparison. The Langmuir et al., 1992 model does not provide a good fit to any of the Siqueiros plagioclase compositions.

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54 35 40 45 50 55D22-3, spinel 2100*Cr/(Cr + Al) 36 38 40 42 44 46 48 50D22-3, spinel 4100*Cr/(Cr + Al) 23 24 25 26 27D20-8, spinel 1100*Cr/(Cr + Al) 30 35 40 45 50 55 020406080100D21-1, spinel 1100*Cr/(Cr + Al)CORERIM Figure 4-13. Spinel Cr# for co re, interior, and rim locations.

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55 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 00.20.40.60.81Fe3+/(Fe3++Cr+Al)Fe2+/Mg+Fe2+FeTi trend Figures 4-14. Fe3+/(Cr + Al + Fe3+) vs. Fe2+/(Mg + Fe2+) plots for tholeiitic basalts. Fields are from Barnes & Roeder, 2001 and enclose 50% (dark shading) and 90% (light shading) of the MORB data points.

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56 Rim of large spinel Interior of large spinel Core of large spinel Rim of medium-small spinel Interior of medium-small spinel Core of medium-small spinel 0 0.2 0.4 0.6 0.8 1 00.20.40.60.81Cr/(Cr + Al)Fe2+/(Mg + Fe2+) Cr Al trend Figure 4-15. Cr/(Cr + Al) vs. Fe2+/(Mg + Fe2+) plot for tholeiitic ba salts. Fields enclose 50% (dark shading) and 90% (light shading) of the MORB data points. MORB fields are from Barnes & Roeder, 2001.

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57 given Cr/(Cr + Al) when compared to othe r MORB spinels (Figure 4-15). Comparison of the host-glass compositions with spinel co mpositions shows a str ong correlation between the Al content of spinel rims and glass, but not for core and interior compositions (Figure 4-16). Similar correlations between Al cont ent of the host rock and spinel have been found in other MORB lavas (Sigurdsson a nd Schilling, 1976, Allan et al. 1988, Dick and Bullen, 1984). A strong correla tion also exists between the Mg# of the host glass and the Mg# of the spinels, with the strongest corre lation observed for rim compositions (Figure 4-17). The Cr# of the Siqueiros spinels is independent of the hos t MgO content (Figure 4-18) as found in the Lamont Seamounts (Allan et al., 1988) but contrary to the results of Irvine (1976). Some small sp inels are present in the groundm ass glass. These show Mg# and Cr# correlations similar to those observed in the larger spinels att ached or enclosed in olivines (Figure 4-19). Ther e was no correlation between glass Mg# and spinel Cr# for either type of spinel phe nocrysts (Figure 4-20).

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58 6 7 8 9 10 11 12 8.599.51010.511 y = -30.824 + 3.9306x R2= 0.75193 Spinel Mole % AlGlass Mole % Al Core of spinel Interior of spinel Rim of spinel Figure 4-16. Molecular percen tage aluminum in glass versus molecular percentage aluminum in spinel. There is a fairly linear relationship for rim compositions of the spinels, but the there is less of a relationship for core and interior compositions. 0.6 0.65 0.7 0.75 0.8 0.60.620.640.660.680.70.72 y = -0.45953 + 1.736x R2= 0.9263 Spinel Mg#Glass Mg# Core of spinel Interior of spinel Rim of spinel Figure 4-17. Comparison of the composition of the cores, interiors, and rims of spinels found in the groundmasses and within o livines with the composition of the host glass. A strong corr elation can be seen betw een spinel and glass Mg#.

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59 20 25 30 35 40 45 50 55 60 0.620.640.660.680.70.72Spinel Cr/(Cr+Al)Glass Mg# Core of spinel Interior of spinel Rim of spinel Figure 4-18. Comparison of the composition of the cores, interiors, and rims of spinels found in the groundmasses and within o livines with the composition of the host glass. There is a poor correlati on between host glass an d spinel Cr/(Cr + Al). Spinel inside or attached to olivine Spinel in glass 0.6 0.65 0.7 0.75 0.8 0.60.620.640.660.680.70.72 y = -0.35923 + 1.5788x R2= 0.85569 y = -0.42799 + 1.7013x R2= 0.85135 Spinel Mg#Glass Mg# Figure 4-19. Comparison of the composition of the spinels found inside olivines and spinels found in the glass with the co mposition of the host glass. A strong correlation between spinel and glass M g# exists for both types of spinels.

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60 20 25 30 35 40 45 50 55 60 0.620.640.660.680.70.72Spinel Cr/(Cr + Al)Glass Mg# Spinel inside or attached to olivine Spinel in glass Figure 4-20. Comparison of the composition of the spinels found inside olivines and spinels found in the glass with the co mposition of the host glass. There is poor correlation between host glass Mg# and spinel Cr/(Cr + Al) for both types of spinels.

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61 CHAPTER 5 MAJOR AND TRACE ELEMENT CHEMISTRY Major Element Trends The major element compositions of Siqueiros basalt samples are presented in Appendix C and are shown in Figures 5-1 and 5-2. For comparative purposes, the samples in Figures 5-1 and 5-2 are divided into groups according to the geologic setting from which they were recovered. Basalt samp les analyzed in this study include samples from the three spreading cen ters (A, B, and C), trough D, the 3 connecting transform faults (A-B, B-C, and C-D), the western ri dge transform intersection (WRTI), and the eastern ridge transform intersection (ERTI). The lavas from the Siqueiros transform domain can be classified as tholeiitic basalts having low K2O and total alkali contents as well as showing FeO enrichment trends with de creasing MgO; characteri stic of tholeiitic suites (Tilley, 1950). The MORB recovered incl ude picrites, picritic basalts, basalts, ferrobasalts, and a few Feand Ti-enriched (FeT i) basalts. Classifi cation of a ferrobasalt is defined as containing gr eater than 12 wt. % FeO, but less than 2 wt. % TiO2, while FeTi basalts are defined as containing greater than 12 wt. % FeO and TiO2 contents greater than 2 wt. % The MORB can be further divided into N-MORB (normal, incompatible element-depleted mid-ocean ridge basalts), D-MORBs (exceptionally depleted, incompatible element-depleted mid-ocean ridge basalts), E-MORB (incompatible element-enriched mid-ocean ridge basalts), and T-MORB (mid-ocean ridge basalts transitional be tween N-MORB and E-MORB). There is currently some debate over the nomenclature of high-MgO volcanic rocks (Kerr and Arndt, 2001;

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62 1 1.5 2 2.5 3TiO2 2.2 2.4 2.6 2.8 3 3.2Na2O 47 48 49 50 51 567891011SiO2 MgO 567891011 9 10 11 12 13CaOMgO Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 5-1. Major element variation diagrams for glasses from the Siqueiros transform domain. Samples are distinguished accor ding to their geologic locations within the transform. Picr itic basalts and picrites are not shown on this diagram.

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63 7 8 9 10 11 12 13FeO 0.1 0.2 0.3 0.4 0.5P2O5 12 13 14 15 16 17 18 567891011Al2O3MgO 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 567891011K2O MgO Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 5-1. Continued.

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64 0 0.05 0.1 0.15 0.2 0.25 0.3 567891011MnOMgO 0 0.1 0.2 0.3 0.4 0.5Cr2O3 Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 5-1. Continued.

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65 1 1.5 2 2.5 3TiO2 1.8 2 2.2 2.4 2.6 2.8 3 3.2Na2O 46 47 48 49 50 51 5101520SiO2 MgO 8 9 10 11 12 13 5101520CaOMgO Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Picrites and Picritic Basalts Figure 5-2. Major element variation diagrams showing the Siqueiros picrites and picri tic basalts relative to more evolved MORB as in Figure 5-1. Picrites and picr itic basalts were only recovered within the A-B fault.

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66 7 8 9 10 11 12 13FeO 0.1 0.2 0.3 0.4 0.5P2O5 12 13 14 15 16 17 18 5101520Al2O3 MgO 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 5101520K2OMgO Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Picrites and Picritic Basalts Figure 5-2. Continued.

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67 0 0.05 0.1 0.15 0.2 0.25 0.3 5101520MnOMgO 0.1 0.2 0.3 0.4 0.5Cr2O3 Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Picrites and Picritic Basalts Figure 5-2. Continued.

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68 Le Bas, 2001). The IUGS classification (L e Bas, 2001) for a picrite is >18 wt% MgO with between 1 and 2 wt% total alkalis. This definition is based entirely on the chemistry of the rock. Others (Kerr and Arndt, 2001) advocate for a de finition that places greater emphasis on the texture of the rock, which reflec ts the conditions of crystallization. Such definitions require an abundance of olivine phenocr ysts in order for a rock to be classified as a picrite. Of the rocks analyzed for this study, only two can be clas sified as picrites by the IUGS classification. These two rocks are also rich in ol ivine phenocrysts and fit into the textural definition of a picrite. The term picritic basalt is used to describe highly magnesian rocks that are also olivine phyric but have MgO conten ts too low (12-18 wt% MgO) to be classified as pi crites. The picrites and picritic basalts have only been found within the A-B fault. Where trace element analyzes ar e not available the ratio of K2O to TiO2 can be used as a proxy to discriminate between deplet ed (Ce/Yb < 1) and enriched (Ce/Yb > 1) samples (Perfit et al., 1994.) In the database for the 9 -10 N segment of the EPR there is a natural break at K2O/TiO2 = 0.11. Samples with K2O/TiO2 values < 0.11 are considered N-MORB, which have normal in compatible element-depleted signatures (Smith et al., 2001). The majority of samples recovered from the 9 -10 N area are NMORB, but a small percentage (~15%) of samples found off-axis (300-500 m) have values > 0.11 and are transitional to incomp atible element enriched basalt (T-MORB or E-MORB). For the 11-12 segment of the EPR, the depleted versus enriched sample break was found to correlate with a K2O/TiO2 value of 0.25 (Hekinian et al., 1989). The K2O/TiO2 ratios of the Siqueiros samples are sh own in Figure 5-3. In the Siqueiros samples there is a break between K2O/TiO2 values < 0.10 and K2O/TiO2 values greater

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69 0 0.1 0.2 0.3 0.4 0.5 56789101112K2O/TiO2MgO Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection EPR Field Figure 5-3. Comparison of K2O/TiO2 of Siqueiros samples with samples from the EPR. A K2O/TiO2 > 0.11 indicates an enriched sample. The Siqueiros samples are very de pleted when compared to the EPR field.

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70 than 0.15. All of the samples with K2O/TiO2 values greater than 0.15 are from the RTI and the group includes sample 2390-1, which is well documented to be incompatible element enriched as well as to have higher 87Sr/86Sr than all other MORB from the region (Lundstrom et al., 1999). For the Siqueiros sample suite, K2O/TiO2 values 0.11 correlate with depleted Ce/Yb ratios (Discussed in Trace Element Trends section). Unlike the EPR, none of the Siqueiros samples have transitional K2O/TiO2 values between 0.15 and 0.25 (Figure 5-3). Compar ed to the EPR, Siqueiros samples are significantly more depleted and include very few enriched samples. Only the samples from spreading center B that have higher K2O/TiO2 values and the enriched RTI samples overlap with the EPR field. The Siqueiros samples have a narrow ra nge in MgO content with a relatively primitive average of 8.31 wt. % MgO. Th e most primitive MORB are found within the A-B fault (MgO contents of ~ 10-10.5 wt. %), but were recovered ne ar basalts that had MgO contents of ~7 wt. %. The most e volved MORB are found n ear the RTIs (MgO contents of 5.38 wt. %) and were recovered with samples that have as much as ~8 wt. % MgO. The most primitive samples have FeO, and TiO2 contents (in wt. %) of 7.13% and 0.93%, respectively and the most e volved samples have FeO and TiO2 contents of 12.79%, and 2.85%, respectively. Most of the variation seen in major elem ents on the segment scale is due to lowpressure crystallization in shallow magma ch ambers (Perfit et al., 1983; Langmuir et al., 1992; Batiza and Niu, 1992). Low-pressure crys tallization results in changes in melt compositions that vary systematically with MgO, which has been shown to decreases during cooling due to the removal of olivin e from the melt (Langmuir et al., 1992). The

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71 basalts from the Siqueiros transform exhibit major element variations that appear to mainly reflect the effects of crystal fractiona tion (Figures 5-1 and 52). In the Siqueiros sample suite, FeO and Na2O contents steadily increase with decreasing MgO content. P2O5, K2O, and TiO2 also increase with decreasing MgO, but to greater relative extents. MnO and SiO2 show a little more scatter, but al so generally increase with decreasing MgO. Cr2O3 decreases with decreasing MgO and CaO and Al2O3 both initially increase and then decrease with decreasing MgO. Thes e variations are compatible with initial olivine fractionation, followed by plagioclas e fractionation, and finally clinopyroxene fractionation (Batiza et al ., 1977; Perfit et al., 1996). Comparison of the 3 spreading centers show s that the major element variations of lavas from the 3 spreading centers are genera lly very similar. Spreading center B does contain samples that are slightly more evol ved (MgO < 7%) and has some samples with slightly higher TiO2, Al2O3, P2O5, and K2O for a given MgO when compared to the other spreading centers and the faults. Spreading center A has a group of samples with lower Na2O for a given MgO content when compared with all other Siqueiros samples (Figure 5-2). The most primitive samples were recovered from the A-B fault and all of the picritic basalts were found within this fault. The other faults, B-C and C-D, also contain some samples that are relatively primitive in comparison to the spreading centers. When compared to the other localities within the Siqueiros transform, samples from the B-C fault have low TiO2, K2O, FeO, and P2O5 values. Fault B-C has the most evolved samples of the 3 intra-transform shear zones. The most evolved samples from the entire transform domain were recovered along the west ern ridge transform intersection (WRTI). A subset of these samples from the WRTI ar e very different and do not appear to be

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72 related by fractional crystallization to the othe r Siqueiros samples. Compared to all other samples from the Siqueiros transform, these RTI samples have higher Al2O3 and lower P2O5, K2O, Na2O, and CaO contents for a given MgO. The samples from spreading center B, which exhibits the most symmetric spreading pattern, were compared to determin e whether or not there is symmetry of lava composition about the axis and to determine if there is a systematic change in lava chemistry with time (distance from axis) for the intra-transform spreading centers. As basalts are carried off-axis by spreading, they record the chemical composition of the axial melt lens at the time they were erupt ed. If the basalts are not buried beneath younger, off-axis flows, the distribution of lava compositions may show systematic differences in magma chamber chemistry with time. Spreading center B was chosen because it is the most well sampled and it appears to be the most well developed spreadi ng center in the Siqueiros transform. Morphological symmetry of ridges can be iden tified up to 30-40 km from the axis of spreading. This suggest that spreading center B has been active longer than the other spreading centers, which only exhibit symmetry 10-20 km from the spreading centers. Variations in MgO content and depth to s eafloor were compared with the sample’s distance from the axis of B (Figure 5-4). Sa mples recovered from the axis have a wide range in MgO contents (~7-8.5 wt. %). Most of the samples with the highest MgO contents were found within the axis, bu t high MgO samples were also found on the western side. The most evolved samples were found furthest east from the axis. Smooth fit lines of depth and MgO va riation with distance from the axis show no correlation between sample depth and MgO content. Ba sed on the samples recovered, there does not

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73 appear to be a symmetrical variation in MgO contents or any systematic change in lava chemistry with distance or depth, but sampling is too sparse off axis to truly evaluate this. Comparison of Siqueiros Samples to the Adjacent EPR and Garrett Transform MORB mantle compositional heterogeneit ies exist on various scales: global, regional and local. Local variability can ex ist on segment and sub-segment scales with compositional variations along and across axis for individual segments and even within individual lava flows (Perf it & Chadwick, 1998). In order to better understand global and regional variability, the major element contents of the Siqueiros samples were compared to that of the adjacent EPR and the Garrett transform in order to determine how the samples relate to the regional chemistry of the EPR and the chemistry of lavas erupted in other fast slipping transforms (Figures 5-5 and 5-6). The major element compositions of Siqueiros samples can be compared to the well studied 9-10 N segment of the EPR, which is directly north of the transform. Most of the Siqueiros samples from the 3 intra-transform sp reading centers fall within the EPR fields. Siqueiros samples from the faults are less evolved and slightly more depleted in K2O and P2O5 than the EPR samples with the picrites a nd the picritic basalts from the A-B fault and some of the samples from the C-D fault being particularly less evolved than the EPR lavas. A few of the samples from the RTIs fall outside the EPR fiel d and are unlikely to be related to the EPR samples by fractional crystallization. Siqueiros samples from the 3 intra-transf orm spreading centers and transform shear zones are similar to Garrett transform basalts, but have slightly lower P2O5, K2O, and TiO2 contents. Lavas from spreading center A, which were found to be depleted in Na2O

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74 6.5 7 7.5 8 8.5 9-20000 -15000 -10000 -5000 0 5000 10000 15000 20000MgODistance from B axis (m) 2000 2200 2400 2600 2800 3000 3200Depth Figure 5-4. MgO (wt. %) and depth to seafloor versus distance from the axis of spreading center B. Negative values ar e west of the axis and positive values are east of the axis. Th ick line is smooth fit trend.

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75 1 1.5 2 2.5 3TiO2 2.5 3 3.5Na2O 47 48 49 50 51 567891011SiO2MgO 9 10 11 12 13 567891011CaO MgO Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection EPR Field Figure 5-5. Variation diagrams co mparing Siqueiros lava compositi ons with basalts from the 9-10 N segment of the EPR (Perfit e t al., personal communication).

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76 7 8 9 10 11 12 13FeO 0 0.1 0.2 0.3 0.4 0.5 0.6P2O5 12 13 14 15 16 17 18 567891011Al2O3 MgO 567891011 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7K2OMgO Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection EPR Field Figure 5-5. Continued.

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77 1 1.5 2 2.5 3TiO2 2 2.5 3 3.5 4Na2O 46 47 48 49 50 51 567891011SiO2MgO 567891011 9 10 11 12 13CaO MgO Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Garrett Field Figure 5-6. Variation diagrams comparing the compositions of th e Siqueiros and Garrett samples (from Hekinian et al., 1995).

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78 7 8 9 10 11 12 13FeO 0.1 0.2 0.3 0.4 0.5 0.6P2O5 12 13 14 15 16 17 18 567891011Al2O3MgO 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 567891011K2OMgO Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Garrett Field Figure 5-6. Continued.

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79 compared to other Siqueiros samples, are also slightly depleted in Na2O compared to the Garrett samples. On average the RTI samples are more evolved than the Garret samples and are more enriched in K2O and P2O5. Trace Element Trends The trace element contents of the Siqueiros samples are presented in Appendix D. Selected trace elements were plotted against TiO2 and Zr, which both behave incompatibly during crystal fractionation and mantle melting (Figures 5-7 and 5-8). Incompatible element contents generally increase with increasing fractional crystallization, while compatible element cont ents decrease with magmatic evolution. The trace elements Ni and Cr behave compa tibly exhibiting relatively coherent trends that decrease with increasing TiO2 and Zr. Ni is compatible in olivine and initially decreases with fractionation. Cr is compatible in spinel and olivin e and exhibits a very high initial decrease with fractionation. Y, V, and Zr all behave incompatibly and increase smoothly with increasing magma e volution. Sr, which is compatible in plagioclase, initially incr eases with increasing TiO2 and Zr, but then levels off increasing only slightly with further magmatic differentiation. In general the samples show a narrow range in total abundance for each trace element except for the RTI samples. Samples 2390-1, 2390-3A, 2390-3B, 2390-4, 23905, and 2390-8 are enriched in th e trace element Sr and depleted in Y. Samples 2390-1 and 2390-5 are also slightly enriched in Ni wh en compared to the other evolved samples. These samples from the RTI do not appear to be related to the other samples by fractional crystallization and are classified as E-MO RBs based on their Ce/Yb ratios. Samples RC41 and D30-1 (both from the ERTI) and sample 2390-9 (from the WRTI) do not group with the other samples from the RTI, but appe ar more similar to those from the spreading

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80 200 400 600 800 1000 1200 1400 1600 0.511.522.53Cr TiO2 50 100 150Zr Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 5-7. Trace elements versus TiO2. Ti behaves incompatibly and increases with magma evolution. Trace element da ta from XRF and DCP analysis.

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81 8 16 24 32 40 48 56 64 0.511.522.53Y TiO2 200 400 600 800Ni Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 5-7. Continued.

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82 80 120 160 200 240 280 320 0.511.522.53Sr TiO2 Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 5-7. Continued.

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83 406080100120140160180200 400 800 1200 1600Cr Zr 200 250 300 350 400 450 500V Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 5-8. Trace elements versus Zr. Zr behaves incompatibly and increases with magma evolution. Trace element da ta from XRF and DCP analysis.

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84 8 16 24 32 40 48 56 64 406080100120140160180200Y Zr 200 400 600 800Ni Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 5-8. Continued.

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85 50 100 150 200 250 300 350 406080100120140160180200Sr Zr Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 5-8. Continued.

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86 centers and faults. Samples from spreading center B show a slightly greater range in abundance of the trace elements than the sa mples from the other spreading centers and faults. A few samples from spreading cente r A show a slight depletion at given TiO2 and Zr values in Sr contents re lative to other moderately evolved samples found within the Siqueiros transform. The relative enrichment factors for the tr ace elements Nb, Sr, Zr, and Y, which have been precisely determined by XRF, we re computed by dividi ng the highest value by the lowest value for each morphotectonic locatio n in the transform domain (Table 5-1). The most incompatible element (Nb) show s the greatest relative enrichment for each location with the exception of the C-D fault in which trace element analysis was only completed for two samples from the fault. Nb and Zr, the most incompatible elements, show a lower enrichment than Y and Sr in the two C-D samples. This is because the REE patterns of these samples cross a nd they cannot be related by fractional crystallization. Sr, the most compatible el ement, generally shows the least amount of enrichment for each location. Table 5-1. Nb, Sr, Zr, and Y enrichment f actors for Siqueiros transform morphotectonic locations. Enrichment Factors Nb Zr Y Sr Spreading Center A 3.00 2.05 1.63 1.73 A-B Fault 6.59 3.75 3.35 2.74 Spreading Center B 2.72 2.13 2.10 1.53 B-C Fault 2.25 2.33 1.29 1.24 Spreading Center C 3.85 1.66 1.39 1.29 C-D Fault 1.07 1.05 1.24 1.22 Trough D 1.38 1.15 1.05 1.19 Ridge Transform Intersection* 1.79 1.82 1.77 1.13 *E-MORBs not included in calculation of RTI enrichment factors.

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87 As previously discussed, the K2O/TiO2 ratio has been used as a proxy for degree of enrichment based on comparison with (Ce/Yb) ra tios. The (Ce/Yb) ratio compares a light rare earth element (LREE), Ce, to a hea vy rare-earth element (HREE), Yb. When normalized to chondrites a (Ce/Ybn) ratio of 1 indicates no enri chment or depletion of the LREE to HREE relative to chondrites. A (Ce/Ybn) ratio greater than one indicates that a sample is LREE enriched and a (Ce/Ybn) ratio less than one i ndicates LREE depletion. The (Ce/Ybn) values can be used to determine what K2O/TiO2 value matches the boundary between enriched and depleted samples. The measured K2O/TiO2 value of a sample can then be used to classify samples in which trace element analysis has not been completed as enriched or deplet ed. Figure (5-9) shows the (Ce/Ybn) vs. K2O/TiO2. All samples except for the RTI samples are deplet ed compared to chondr ites. Sample 2390-9 has a (Ce/Ybn) ratio about equal to one and samples 2390-1, 2390-5, and 2390-3B are enriched compared to chondrites. The (Ce/Ybn) break in enriched vs. depleted samples is roughly at about K2O/TiO2 = 0.11. Consequently, most of the Siqueiros samples are classified as depleted and only a few samples from the RTI show an enriched signature. The samples from the spreading centers and the faults for the most part fall into narrow (Ce/Ybn) groups based on their location (Fi gure 5-10). The samples from spreading center B and the C-D fault have higher (Ce/Ybn) ratios indicating that they are more enriched overall than the other Siqueiros samples. Sa mples from the A-B fault and spreading center A (except for a few sa mples) have the most depleted (Ce/Ybn) values. Of all the samples for which trace element analysis was done only sample 2390-9 exhibits transitional trace element characteristics.

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88 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.511.522.53 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35K2O/TiO2 Ce/Yb (chondrite normalized) Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 5-9. Ce/Ybn vs. K2O/TiO2 of the Siqueiros samples.

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89 0 0.05 0.1 0.15 0.2 0.50.60.70.80.911.1 0 0.05 0.1 0.15 0.2K2O/TiO2 Ce/Yb (chondrite normalized) Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 5-10. Chondrite normalized Ce/Yb ra tios for Siqueiros morphotectonic locations.

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90 The samples chosen for ICP-MS analysis range from strongly, light rare earth element depleted to moderately enriched. Mo st of the samples from the transform faults and the spreading centers generally have 620x chondritic values for all REE (Figure 511). The samples from the spreading cente rs and the faults all show strong LREE depletion. Samples from the A-B fault a nd sample 2382-10WR from spreading center B show the strongest depletions in the LREE, having enrichments as low as 2.5x chondritic values. Samples from the RTI contain two groups. The first group contains the EMORBs which are enriched in the LREE (samples 2390-8, 2390-5, 2390-4, 2390-1, and 2390-3) with enrichments up to 60x chondritic values, while enrichments in the HREE are only 20x chondritic values. The REE patte rns of the E-MORB samp les cross those of the “normal” samples and cannot be related by fractional crystalliz ation to any other samples found in the Siqueiros transform. The other group has REE patterns parallel to samples from the spreading centers and fa ults, yet more enriched overall in REE indicating that they are more fractionated. This group includes sample 2390-9, which is classified as a FeTi basalt. It is the mo st evolved sample foun d within the Siqueiros transform domain and the REE patterns parall el those from the spreading centers. Spreading center B, which has the most di fferentiated samples (lower MgO contents) other than the RTI FeTi basalt, has samples with higher REE abundances than any of the other Siqueiros spreading cente rs or faults. The most evolved samples from spreading center B (highest REE abundances) have well developed negative Eu anomalies. Eu anomalies are absent in samples from the ot her spreading centers a nd the faults, but can be found in sample 2386-5 from tough D a nd sample 2390-9, the FeTi basalt from the RTI. Samples 2391-10wr and 2391-5 from the A-B fault show positive Eu anomalies.

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91 2 4 6 8 10 30 LaCeNdSmEuGdDyYErYbLuSpreading Center A 2389-1 D38-2 D36-3 D35-4 2383-6 G b 2389-4 2389-5 2389-5 G 2383-6 2383-6 G aSample/Chondrite 2 4 6 8 10 30 LaCeNdSmEuGdDyYErYbLuA-B Fault D17-1WR 2384-6 G 2384-9 G 2379-2WR 2388-10 G A25 D17-9 D23-2 D20-5 D20-1 D20-15 2384-3 2384-9 2384-1Sample/Chondrite 2 4 6 8 10 30 LaCeNdSmEuGdDyYErYbLuSpreading Center B 2382-10WR 2376-8 2377-11 2377-1 G 2377-8 G 2380-12 G 2380-11 2389-5 G 2375-7Sample/Chondrite Figure 5-11. Chondrite normalized REE diagrams Representative samples were chosen for each morphotectonic location. REE an alysis by ICP-MS at the University of Houston and by ICP-MS at the Geol ogical Survey of Canada. Chondritic normalization factors are from Sun and McDonough, 1989.

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92 2 4 6 8 10 30 LaCeNdSmEuGdDyYErYbLuB-C Fault 2381-11 2387-5 2381-3AWR 2387-6Sample/Chondrite 2 4 6 8 10 30 LaCeNdSmEuGdDyYErYbLuSpreading Center C D33-1 2378-6 2378-2 G 2378-3 G 2378-8 G 2385-2 G 2385-3B GSample/Chondrite 2 4 6 8 10 30 LaCeNdSmEuGdDyYErYbLuC-D Fault D27-5Sample/Chondrite Figure 5-11. Continued.

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93 4 6 8 10 30 LaCeNdSmEuGdDyYErYbLuTrough D 2386-5 D44-1 2386-5 GSample/Chondrite 4 6 8 10 30 50 LaCeNdSmEuGdDyYErYbLuRidge Transform Intersection D30-1 2390-3B 2390-5 G 2390-7 2390-9 2390-1 2390-3A 2390-4 2390-5 2390-8Sample/Chondrite Figure 5-11. Continued.

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94 Other than the more depleted samples from the A-B fault and E-MORB samples from the RTI, samples from the spreading centers and fa ults generally have parallel REE patterns. A few of the samples do have patterns that are slightly different and cross other REE patterns (eg. samples 2389-1 and 2383-2 from sp reading center A) in dicating that more than one “normal” parental composition is required to produce all samples within the Siqueiros transform. The (La/Sm)n values for the entire suite ra nge from 0.289 (sample 2384-1 from the A-B fault) to 1.98 (sample 2390-1 from the RT I) and have an median value of 0.596. The (Sm/Yb)n values range from 0.649 (s ample 2391-5 from the A-B fault) to 1.803 (sample 2390-5G from the RTI). This indicates that the Sm/Yb portion of the rare-earth element curves are flatter than the La-Sm por tion and that the samples are more depleted in the LREE. N-MORB normalized REE diagrams are shown in figure 5-12. Samples from spreading centers A and C are fa irly flat and parallel to each other. The samples are all close to N-MORB values with a range of 0.2x – 2x N-MORB. All samples from spreading center B, except for sample 238310WR, have REE values that are overall enriched compared to N-MORB. Samples from fault C-D and trough D are flat, but overall are slightly depleted when compared to N-MORB. The B-C and A-B faults are also slightly depleted when compared to N-MORB, but have stronge r depletions in the LREE then the HREE. The A-B fault is the most depleted in the LREE. The samples from the RTI range from parallel to and slight ly enriched in the LREE to enriched in the LREE.

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95 0.2 0.4 0.6 0.8 1 LaCeNdSmEuGdDyYErYbLuSpreading Center A 2389-1 D38-2 D36-3 D35-4 2383-6 G b 2389-4 2389-5 2389-5 G 2383-6 2383-6 G aSample/N-MORB 0.2 0.4 0.6 0.8 1 LaCeNdSmEuGdDyYErYbLuA-B Fault D17-1WR 2384-6 G 2384-9 G 2379-2WR 2388-10 G A25 D17-9 D23-2 D20-5 D20-1 D20-15 2384-3 2384-9 2384-1Sample/N-MORB 0.2 0.4 0.6 0.8 1 LaCeNdSmEuGdDyYErYbLuSpreading Center B 2382-10WR 2376-8 2377-11 2377-1 G 2377-8 G 2380-12 G 2380-11 G 2389-5 G 2375-7Sample/N-MORB Figure 5-12. N-Morb normalized REE diagra ms. Representative samples were chosen for each morphotectonic location. REE an alysis by ICP-MS at the University of Houston and by ICP-MS at the Geological Survey of Canada. N-MORB normalization factors are from Sun and McDonough, 1989.

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96 0.2 0.4 0.6 0.8 1 LaCeNdSmEuGdDyYErYbLuB-C Fault 2381-11 G 2387-5 2381-3AWR 2387-6Sample/N-MORB 0.2 0.4 0.6 0.8 1 LaCeNdSmEuGdDyYErYbLuSpreading Center C D33-1 2378-6 2378-2 G 2378-3 G 2378-8 2385-2 G 2385-3B GSample/N-MORB 0.2 0.4 0.6 0.8 1 LaCeNdSmEuGdDyYErYbLuC-D Fault D27-5Sample/N-MORB Figure 5-12. Continued.

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97 0.2 0.4 0.6 0.8 1 LaCeNdSmEuGdDyYErYbLuTrough D 2386-5 D44-1 2386-5 GSample/N-MORB 1 10 LaCeNdSmEuGdDyYErYbLuRidge Transform Intersection D30-1 2390-3B 2390-5 G 2390-7 2390-9 2390-1 2390-3A 2390-4 2390-5 2390-8Sample/N-MORB Figure 5-12. Continued.

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98 It is important to note that the majority of the N-MORB samples have lower K/Ti ratios than typical MORB from the northern EP R. This was also noted for samples from the Garrett transform relative to lavas from the southern EPR. All of the low K/Ti Garrett samples were categori zed as “D-MORB” (Hekinian et al., 1995). The K/Ti ratio by itself is not an adequate characteristic to distinguish D-MORB from N-MORB in the Siqueiros sample suite because the REE pattern s of many of the low K/Ti samples are not sufficiently depleted in LREE to be “deplete d” relative to NMORB (Figure 5-12). Only Siqueiros lavas from the A-B fault with unus ually depleted LREE pa tterns are considered “DMORB.” When normalized to the N-MORB value of Sun & McDonough (1989) some of the other samples from the Siqueiros transform domain are depleted in the LREE and have Ce/Y (N-MORB normalized) valu es < 1.0, however, only samples from the highly depleted samples from the A-B fau lt were classified as D-MORB. This corresponds to a Ce/Y N-MORB normalized value of < 0.80 (Figure 5-13). When normalized to E-MORB values, the Siqueiros E-MORB are slightly more evolved with REE values around 2x that of E-MORB abundances (Figure 5-14). The Siqueiros E-MORB are also slightly more enriched in the LREE. The Siqueiros EMORB patterns all parallel each other indicating that they could be derived from a common parental composition. There is also very little difference in REE abundances indicating little difference in fractional cr ystallization history be tween the Siqueiros EMORB samples. Primitive mantle normalized trace element diagrams are shown in figure 5-15. All of the Siqueiros samples other than the E-MORB found at the RTI have low concentrations of incompatible elements a nd large-ion-lithophile elements. Samples

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99 2388-10G and 2384-9G are exceptionally deplet ed in the large-ion-lithophile and incompatible elements. Many of the samples show negative Sr anomalies. E-MORB incompatible element abundances indicate that they do not share a common parental magma with the other Siqueiros samples.

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100 Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection 0.6 0.8 1 1.2 1.4 00.511.522.53 0.6 0.8 1 1.2 1.4Ce/Y (N-MORB normalized)Ce (N-MORB normalized) Figure 5-13. N-MORB normalized Ce/Y ratio s for Siqueiros transform morphotectonic locations. E-MORB samples from the WRTI are not included. 0.1 1 10 LaCeNdSmEuGdDyYErYbLuRidge Transform Intersection 2390-1 2390-3A 2390-4 2390-5 2390-8 2390-3B 2390-5 GSample/E-MORB Figure 5-14. REE diagram of RTI E-MORBs plotted relative to E-MORB values. REE analysis by ICP-MS at the University of Houston and by ICP-MS at the Geological Survey of Canada. E-MO RB normalization factors are from Sun and McDonough, 1989.

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101 1 10 100 CsRbBaThUNbTaKLaCePbPrSrNdZrHfSmEuTiTbDyYHoErYbLuSpreading Center A 2383-6 G b 2389-4 2389-5 2389-5 G 2383-6 G aSample/Primitive Mantle 1 10 CsRbBaThUNbTaKLaCePbPrSrNdZrHfSmEuTiTbDyYHoErYbLuA-B Fault 2388-10 G A25 D1-3 2376-5 G 2377-10 G 2380-12 G 2384-9 GSample/Primitive Mantle 1 10 CsRbBaThUNbTaKLaCePbPrSrNdZrHfSmEuTiTbDyYHoErYbLuSpreading Center B A25 D18-2 G 2375-7 G 2376-7 G 2377-10 G 2380-12 GSample/Primitive Mantle Figure 5-15. Primitive mantle-normalized trace element diagrams. Trace element data completed by ICP-MS at the Geological Survey of Canada. Primitive mantle normalizing factors are from Sun a nd McDonough, 1989. Elements are listed from right to left in order of increas ing incompatibility during mantle melting.

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102 1 10 CsRbBaThUNbTaKLaCePbPrSrNdZrHfSmEuTiTbDyYHoErYbLuB-C Fault 2381-11 G 2387-5Sample/Primitive Mantle 1 10 CsRbBaThUNbTaKLaCePbPrSrNdZrHfSmEuTiTbDyYHoErYbLuSpreading Center C 2378-2 G 2378-3 2378-3 G 2378-8 G 2385-2 G 2385-3B GSample/Primitive Mantle 1 10 CsRbBaThUNbTaKLaCePbPrSrNdZrHfSmEuTiTbDyYHoErYbLuTrough D 2386-5 GSample/Primitive Mantle Figure 5-15. Continued.

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103 1 10 CsRbBaThUNbTaKLaCePbPrSrNdZrHfSmEuTiTbDyYHoErYbLuRidge Transform Intersection 2390-3B 2390-5 GSample/Primitive Mantle Figure 5-15. Continued.

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104 CHAPTER 6 PETROGENESIS Major Element Models Liquid lines of descent were calcul ated using the pr ogram PETROLOG 2.1 (Danyushevsky et al., 1996), which determines the major element contents of sequential residual liquids by fractional crystallization of a given parental composition (Appendix E). The program calculates liquidus minerals and temperatures fo r the range of melt compositions. The minerals on the liquidus are incrementally removed and residual liquid compositions are re-calculated. The chemistry of these residual liquids determine the liquid line of descent for a given parent The liquid lines of de scent are theoretical magmatic evolutionary paths that show how the magma co mposition might change with decreasing temperatures and pr ogressive fractionation. The program input requires an initial composition, the H2O content of the initial composition, the pressure of cr ystallization, the amount of crys tallization between steps, and the potential minerals in e quilibrium during crys tallization. Models were run using a variety of samples with relatively high MgO contents as parental compositions. Sample 2377-7, a moderately evolved sample (MgO content of 8.03 wt. %), was also used because of its relative enrichment in some of the more incompatible elements (TiO2, P2O5, and K2O). In order to provide a relativ ely primitive parental composition, the composition of sample 2377-7 was run backward s using reverse crystal fractionation in Petrolog. The reverse crystal fractionation optio n in Petrolog allows chosen minerals to be added until a specified MgO content is reached. Plagioclase and olivine were both

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105 added until a MgO content of 10.5 wt. % was reached. This new composition, 2377-7P, was then used to run the crys tal fractionation liquid line of descent. All other parental compositions were taken directly from the ma jor element microprobe analysis of sample and are designated by the sample number plus th e letter “P” (eg. D34-2P refers to the parental composition based on the major elemen t analysis of sample D34-2). Olivine, plagioclase, and clinopyroxene were chosen as potential mine rals in equilibrium for all samples and spinel was also used for samples for which Cr contents were available. All models were run with 1% melt increments between calculations and were stopped when liquids reached 5 wt. % MgO. The initi al calculations were run under anhydrous conditions and at a pressure of 0.33 kbar to es timate the depth of the melt lens at 1 km (Rosendahl et al., 1976). The Danyushevs ky (2001) olivine, plagioclase, and clinopyroxene crystal fractionation models and the Ariskin & Ni kolaev (1996) spinel crystal fractionation model were chosen. Under low pressure, the parental liquids begin to first crysta llize at about 12601380C. Spinel is the first mineral to crys tallize. Only a small amount of spinel crystallizes and the liquid cools to 1200-1250C before the onset of olivine crystallization which is followed closely by plagioclase crys tallization. Clinopyroxene does not begin to crystallize until the liquids cool to ar ound 1160-1180C, which corresponds to a MgO contents of 6.8-8.0 wt. % (Figure 6-1). The re sults suggest that nearly 50-55 wt. % total crystallization is required to explain the most evolved samp les from the spreading centers and faults and up 70 wt. % total crystallizati on is required to expl ain the most evolved samples (other than the E-MORB) from the RTI. The crystallization occurs in two steps:

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106 0 10 20 30 40 50 60 70 80 112011401160118012001220124012601280% CrystalTemp (C) Ol + Spinel Ol + Plag Ol + Plag + Cpx 2377-7P D34-2P 2384-9P 2384-9P at 2.5 Kbar Figure 6-1. Percentage of crys tals removed as a function of temperature. Solid lines mark onset of plagioclase fractionation. Double lines mark onset of clinopyroxene fractionation. Top ar row indicates amount of crystal fractionation required in order to mode l the most evolved sample from the RTI and second arrow indicates amount re quired to produce that most evolved samples from the spreading centers and faults.

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107 0 10 20 30 40 50 60 70 802384-9P at 2.5 kbarCumulative % CrystalPlagioclase Cpx Olivine Spinel 0 8 16 24 32 40 48 56 642384-9PCumulative % CrystalCpx Plagioclase Olivine Spinel 0 8 16 24 32 40 48 56 64D34-2PCumulative % CrystalSpinel Olivine Plagioclase Cpx 0 8 16 24 32 40 48 56 64 010203040506070802377-7PCumulative % Crystal% Crystal Olivine Plagioclase Cpx Figure 6-2. Percentage of li quid and removed crystals as a function of percentage of crystals removed from magma for 2 377P, D34-2P and 2384-9P. Fractional crystallization models calculated us ing low pressure model of Danyushevsky (2001). High pressure model of 2384-9P is also shown for comparison.

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108 an initial 18-47 wt. % crystal lization of olivine plus plag ioclase with minor spinel crystallization followed by 8-37 wt. % crysta llization of olivine, plagioclase, and clinopyroxene is predicted by th e models (Figure 6-2). The m odels are in agreement with low pressure experiments completed on sample 2384-1, which predicted initial temperatures of crystallization around 1297-1306C and olivine spinel as liq uidus phases for pressures below 12-13 kbar when a plagioclase lherzolite sour ce is assumed (Wendlandt et al., 1994). In order to explain the major element vari ability observed in the entire suite of samples, three parental compositions were c hosen (Figure 6-3). 2377-7P from spreading center B provides the best fit to the sample s that are more enriched in the slightly incompatible major elements such as TiO2, K2O, and P2O5. D34-2P from the C-D fault is more depleted in the incompatible major elements and has anomalously low Na2O and 2384-9P is one of the high-MgO basalts from the A-B fault and provides a good fit for samples with lower FeO and higher Na2O contents. Most of the samples can be explained by low pressure fractional crystall ization of these three parents (2377-7P, D342P, 2384-9P). However the low pressure, a nhydrous models were unable to account for the lower CaO wt. % samples and high Al2O3 wt. % samples. Higher pressure (2.5 kbars) fractional crystallization of 2384-9P stabi lized clinopyroxene earlier and provided a better fit to the low CaO data (Figure 6-3) High pressure crys tal fractionation could occur at the base of the crust where a sec ond low velocity zone has been identified in geophysical surveys (Solomon and Toomey, 1992) Hydrous calculati ons were also run for 2377-7P, D34-2P, and 2384-9P (Figure 6-4) The hydrous calculati ons were run with 0.15 wt. % H2O, based on the average H2O content reported in the Cameca SX50 electron

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109 47 48 49 50 51 52 456789101112SiO2MgO 1 1.5 2 2.5 3 3.5TiO2 Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection 2377-7P D34-2P 2384-9P 2384-9P at 2.5 Kbar Figure 6-3. Comparison of major element data with LLD models calculated using the olivine, plagioclase, and clinopyroxe ne fractionation models of Danyushevsky (2001). 2377-7P and D34-2P were run at low pressure (<1 kbar) and 2384-9P was run at low pressure (< 1 kbar) and 2.5 kbar.

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110 12 13 14 15 16 17 18 19 456789101112Al2O3MgO 7 8 9 10 11 12 13 14 15FeO Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection 2377-7P D34-2P 2384-9P 2384-9P at 2.5 Kbar Figure 6-3. Continued.

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111 8 9 10 11 12 13 14 456789101112CaOMgO 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6Na2O Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection 2377-7P D34-2P 2384-9P 2384-9P at 2.5 Kbar Figure 6-3. Continued.

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112 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 456789101112K2OMgO 0.1 0.2 0.3 0.4 0.5 0.6P2O5 Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection 2377-7P D34-2P 2384-9P 2384-9P at 2.5 Kbar Figure 6-3. Continued.

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113 47 48 49 50 51 52 456789101112SiO2 MgO 1 1.5 2 2.5 3 3.5TiO2 Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection 2377-7P D34-2P 2384-9P 2384-9P at 2.5 Kbar Figure 6-4. Comparison of major element data with hydrous LLD models calculated using the olivine, plagioclase and clinopyroxene fractionation models of Danyushevsky (2001). Calcula tions include 0.15 wt. % H2O. The hydrous LLD models provide a better fit for the Al2O3 data, but do not considerably change the fractionation trends. 237 7-7P and D34-2P were run at low pressure (< 1 kbar) and 2384-9P was run at low pressure (< 1 kbar) and at 2.5 kbar.

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114 12 13 14 15 16 17 18 19 456789101112Al2O3MgO 7 8 9 10 11 12 13FeO Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection 2377-7P D34-2P 2384-9P 2384-9P at 2.5 Kbar Figure 6-4. Continued.

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115 8 9 10 11 12 13 14 456789101112CaO MgO 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6Na2O Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection 2377-7P D34-2P 2384-9P 2384-9P at 2.5 Kbar Figure 6-4. Continued.

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116 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 456789101112K2O MgO 0.1 0.2 0.3 0.4 0.5 0.6P2O5 Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection 2377-7P D34-2P 2384-9P 2384-9P at 2.5 Kbar Figure 6-4. Continued.

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117 microprobe analysis. The hydrous calculations cause the liquids to crystallize at lower temperatures (1175-1220C) and cause plagiocl ase and clinopyroxene to stabilize a little bit later. The slight delay in the onset of plagioclase crystallization is enough to raise the Al2O3 of the liquid lines of descent. Since the water resulted in later crystallization of clinopyroxene, hydrous calculations were unabl e to account for the low CaO samples. For comparison, liquid line of descent models were also run usi ng the Langmuir et al. (1992) crystal fractionation models for oliv ine, plagioclase, and clinopyroxene. The two models produced almost identical liqui d lines of descent except for the CaO and Al2O3 major elements. The Langmuir modeled liquid lines of descent fall on the high side of the observed CaO data, but provide a better fit to the Al2O3 data without requiring hydrous calculations (Figure 6-5) In order to fit the CaO data, high pressure (2.5 kbars) liquid lines of descen t are still required. For the most part, the Siqueiros major el ement variability can be explained by low pressure crystallization of parental magmas similar in compositi on to 2377-7P, D34-2P, and 2377-7P. In order to expl ain the entire range of Ca O variability, high pressure fractionation is required to stabilize clinopyroxene early, wh ich results in a reduction in CaO in relatively mafic lavas. Al2O3 variability can be explained by either the Langmuir et al. (1992) fractionation m odels or by a small amount of water in the magma as observed in the microprobe data. E-MORB sa mples from the RTI can not be explained by crystal fractionation of the other Siqueiros samples. A parental composition that is more enriched in Al2O3 and more depleted in CaO and FeO is required to explain the EMORB samples.

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118 9 10 11 12 13 456789101112CaO MgO 12 13 14 15 16 17 18 19Al2O3 Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection 2377-7P D34-2P 2384-9P 2384-9P at 2.5 Kbar Figure 6-5. Compar ison of CaO and Al2O3 data with LLD models calculated using the olivine, plagioclase, and clinopyroxene fractionation models of Langmuir et al. (1992). Parental compositions 2377-7 and D34-2 were run at low pressure (< 1 kbar) and 2384-9 was run at low pressure (<1 kbar) and at 2.5 k bars.

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119 Trace Element Models Trace element systematics can be used to better constrain how fractional crystallization and magma mixing have affect ed the chemical evolution of the magmas (e.g., Perfit et al., 1983; Hekinian et al., 1989; Batiza and Niu, 1992). The concentrations of Cr, Sr, Zr, Y, Ni, and V in the residual liquid (Cl) were modeled using the Rayleigh fractionation equation (Cl = Co F (D-1)) (Langmuir et al., 1992). The fraction of melt remaining (F) and the proportions of crystals used in each step were taken from the results of the major element LLD calculatio ns produced in Petrolog for low pressure crystal fractionation (33 ba rs). D34-2P, D20-15P and 2384-9P were chosen as representative parents because they are relatively primitive and have significantly different compositions from one another that might represent different parental melts. The elements chosen to model have a wide ra nge of partition coeffi cients (Kd) for the liquidus minerals observed in the samples (olivine, pl agioclase, clinopyroxene, and spinel). In order to give an idea of th e possible range of compositions that could reasonably be generated by frac tional crystallization, trace element LLDs were produced for the highest and lowest bulk partition co efficients determined for basaltic systems (Villemant et al., 1981; Johnson & Kinzler, 1989; Skulski et al.,1994; Bindeman et al., 1998; Hart & Dunn, 1993; Duke, 1976; Bouga ult & Hekinian, 1974; Ulmer, 1989; Ringwood, 1970; Hauri et al., 1994; Takahashi, 1978; Beattie, 1993; Sun et al., 1974; McKay et al., 1994; Smith, 1993; Perfit et al., 1983; Henderson, 1986; Ragland, 1989; Rollinson, 1993) and are graphically shown in Figure 6-6. Partition coefficients with intermediate values were determined that be st fit the observed data and the results of those calculations are shown in Figure 6-7. Hi gh, low, and best fit partition coefficients are shown in Table 6-1.

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120Table 6-1. List of partition coefficients. Olivine Plagioclase Clinopyroxene Spinel Element High Low Best High Low Be st High Low Best High Low Best Zr 0.06a 0.007b 0.007b 0.27a 0.005b 0.005b 0.27c 0.001d 0.001d0 0 0 Y 0.01e 0.0036b 0.0036b 0.031f 0.02b 0.02b 1.71d 0.29d 0.467g0 0 0 V 0.09h 0.02i 0.02i 0.1j 0h 0h 6.18d 0.22i 1.31k 38l 0 0 Cr 2.1m 0.63b 1n 10m 0h 0h 36i 1.66o 1.66o 77l 5n 5n Ni 48i 2.86p 12n 0.5a 0.01m 0.01m 10i 1.2i 1.2i 10n 10n 10n Sr 0.014q 0.0000154r 0.0000154r10a 1.5l 1.5l 0.449s0.04t 0.04t 0.01n0.01n0.01n Notes: Partition coefficients derived from the following references in the Geoc hemical Earth Reference Model (GERM) at http://www.earthref.org/GERM/main.htm: V illemant et al., 1981a; Johnson & Kinzler, 1989c; Skulski et al.,1994d; Bindeman et al ., 1998f; Hart & Dunn, 1993g; Duke, 1976i; B ougault & Hekinian, 1974j; Ulmer, 1989k; Ringwood, 1970l; Hauri et al., 1994o; Takahashi, 1978p; Beattie, 1993r; Sun et al., 1974s; McKay et al ., 1994t. Other sources not f ound in GERM are Smith, 1993b; Pe rfit et al., 1983h; Henderson, 1986m; Ragland, 1989n; Rollinson, 1993q.

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121 40 60 80 100 120 140 160 180 200 0.511.522.53Zr TiO2 20 30 40 50 60Y Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection D34-2P High Kds D34-2P Low Kds D20-15P High Kds D20-15P Low Kds 2384-9P High Kds 2384-9P Low Kds Figure 6-6. Comparison of observed trace element data with modeled fractionation trends calculated assuming perfect Rayleigh fractional crystallization. Fractionation trends were calculated for parental compositions D34-2P, D2015P, and 2384-9P using the highest and lo west reported partition coefficients for olivine, plagioclase, clinopyroxene, and spinel.

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122 0 100 200 300 400 500 600 0.511.522.53VTiO2 200 400 600 800 1000 1200 1400 1600 1800Cr Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection D34-2P High Kds D34-2P Low Kds D20-15P High Kds D20-15P Low Kds 2384-9P High Kds 2384-9P Low Kds Figure 6-6. Continued.

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123 0 200 400 600 800 1000 0.511.522.53Ni TiO2 50 100 150 200 250 300 350Sr Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection D34-2P High Kds D34-2P Low Kds D20-15P High Kds D20-15P Low Kds 2384-9P High Kds 2384-9P Low Kds Figure 6-6. Continued.

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124 40 60 80 100 120 140 160 180 200 0.511.522.53Zr TiO2 20 30 40 50 60Y Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection D34-2P D20-15P 2384-9P Figure 6-7. Comparison of observed trace element data versus TiO2 with modeled fractionation trends calculated a ssuming perfect Rayleigh fractional crystallization. Fractionation trends were calculated for parental compositions D34-2P, D20-15P, and 2384-9P using the ol ivine, plagioclase, clinopyroxene, and spinel partition coefficients that provided the best fit to the observed trends.

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125 150 200 250 300 350 400 450 500 550 0.511.522.53V TiO2 200 400 600 800 1000 1200 1400 1600 1800Cr Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection D34-2P D20-15P 2384-9P Figure 6-7. Continued.

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126 0 200 400 600 800 1000 0.511.522.53NiTiO2 100 150 200 250 300 350Sr Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection D34-2P D20-15P 2384-9P Figure 6-7. Continued.

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127 Figures 6-6 and 6-7 show the calculated liqui d lines of descent for increments of 5% fractional crystallization for Zr, Y, V, Cr, Ni, and Sr vs. TiO2. In each fractionation step, the percentage of residual liquid has been calculated from the major element LLDs. The parental liquid and fractionating phase compos itions were continually changed after each 5% increment. The fractionating phase com positions were calculated by averaging the phase compositions calculated in the major element variations over the 5% increment. This produces a model in which each parent-to-di fferentiate step is considered a separate event rather than assuming that the differen tiates are a cumulative result of fractionation from one parental composition. The LLDs produced using the highest and lo west available partition coefficients bracket the observed trace element data (except the E-MORB compositions) for all elements other than Zr (Figure 6-6). When intermediate partition coefficients were used, the liquid lines of descent from the three assu med parental liquids fit the observed Ni, Cr, Sr, and V trends well and can explain the a bundances measured in most samples other than the E-MORBS found at the WRTI (Figur e 6-7). The E-MORBs are more enriched in the highly incompatible elements and more depleted in Y and V relative to the “normal samples” and require a very different parent al composition. The li quid lines of descent bracket the Sr data, but pare ntal compositions D20-15P a nd 2384-9P do not fit the Sr data as well as they do the other trace elements. When preparing the samples for XRF analysis care was taken to remove a ll phenocrysts, however, microphenocrysts of plagioclase typically remain and could result in higher Sr contents in some samples which may explain why the Sr contents are sl ightly higher and could explain the slightly higher observed Sr contents than predicted by the models.

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128 The calculated fractional crystallization pa ths for Cr and Ni do not reach some of the most depleted samples using the partition co efficients chosen for the best fit (Figure 6-7). The higher partition coefficients do pr oduce fractionation paths more depleted in Cr and Ni than the observed data (Figure 66), suggesting that the actual partition coefficients for the samples may be between th at chosen for the best fit models and that of the highest parti tion coefficients. Although the Y calculated fractional crysta llization paths bracket the observed Y values, the shape of the fractionation trend doe s not fit the trend of the observed data and the measured Zr values are generally higher th an the calculated tre nds. Zr and Y are both highly incompatible elements and over-enrichments in highly incompatible elements compared to what is calculated by Ralei gh fractionation is comm only observed in MORB suites (Perfit et al, 1983; Bryan et al., 1979). Slightly great er enrichments in Zr and Y can be obtained by modeling the magma system as one batch that crysta llizes to a greater extent (Figure 6-8). In a batch model, th e parental liquid compos ition is not adjusted with each increment and in such a model if any intermediate lavas are removed the more fractionated daughters could not be obtained. It does provi de a method to explain how more evolved liquids enriched in highly in compatible elements could be created, which might mix with other less evolved melts give n the entire range observed in the Siqueiros samples. In summary, calculated trace element vari ations due to fractional crystallization using the parental compositions of D34-2P, D20-15P, and 2384-9P fit the observed data fairly well as shown in trace element-Zr pl ots (Figure 6-9). In order to explain the majority of the samples from the spreading centers and shear zones, between 55-60%

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129 40 60 80 100 120 140 160 180 200 0.511.522.53Zr TiO2 20 30 40 50 60 70 80Y Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection D34-2P D20-15P 2384-9P Figure 6-8. Comparison of observe d trace element data versus TiO2 with modeled fractionation trends calculated a ssuming perfect Rayleigh fractional crystallization. Fractionation trends were calculated as one batch melt in which the parental composition of the derivative magma was not recalculated in 5% increments.

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130 20 30 40 50 60 406080100120140160180200YZr 100 200 300 400 500 600 700 800 900Ni Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection D34-2P D20-15P 2384-9P Figure 6-9. Comparison of observed trace element data versus Zr with modeled fractionation trends calculated a ssuming perfect Rayleigh fractional crystallization. Fractionation trends were calculated for D34-2P, D20-15P, and 2384-9P using the olivine, plagioclase, clinopyroxene, and spinel partition coefficients that provided the be st fit to the observed trends.

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131 0 500 1000 1500 2000 406080100120140160180200CrZr 200 250 300 350 400 450 500 550V Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection D34-2P D20-15P 2384-9P Figure 6-9. Continued.

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132 50 100 150 200 250 300 350 406080100120140160180200SrZr Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection D34-2P D20-15P 2384-9P Figuer 6-9. Continued. fractional crystallization of these parental co mpositions was required. In order to explain the most evolved sample found at th e WRTI, sample 2390-9, over 70% crystal fractionation was required. Parental compositions D20-15P and 2384-9P produce residual liquids slightly more en riched in V and Y for a given Zr content, but this may be due to the under-enrichment of Zr genera ted by the calculations compared to the observed Zr concentrations. None of the m odels can be used to explain the observed concentrations in the E-MORB samples using the N-MORB samples as parental compositions. Overall, parental compositi on D34-2P produces the best liquid line of descent for all the modeled trace elements, but it is clear that more than one liquid line of

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133 descent is needed to fit the entire range of data implying that multiple N-MORB parents are required in the Siqu eiros transform domain. REE Models The rare earth elements (REE) are particularly sensitive to magma genesis and fractional crystallization pro cesses (Perfit et al., 1993). The REE trends show that samples from the A-B fault are more depleted in the LREE than those from the spreading centers and transforms. For this reason, R EE models were produced using D20-15P (AB Fault) and 2375-7P (spreading center B) as parental compositions. The modeled REE trends are shown in figures 6-10 and 6-11. The calculated liquid lines of descent are shown for 10% fractional crys tallization increments. Th e parental composition and fractionating phase compositions were recalculated at 5% increments. Trace elements La, Ce, Sm, Y, and Yb were modeled. The pa rtition coefficients are shown in Table 6-2. The REE patterns modeled by D20-15P are more depleted in the LREE than the REE patterns of samples from the other fault zones and spreading cente rs (Figure 6-11). Fractional crystallization mode ls of D20-15P do provide a cl ose fit to samples recovered in the A-B fault, but some samples from th e A-B fault, such as A25 D17-9, have REE patterns that are better expl ained by fractionation from a parental magma closer in composition to 2375-7P (Figure 6-12). Th e REE pattern modeled by 2375-7P from spreading center B provides a much better f it to the REE patterns observed at the other fault zones and spreading cente rs. 2375-7P is more enriched than other observed compositions which require a more primitive parental composition. The calculations predict that over 60% fractional crystallizati on would be required to produce the most evolved samples from the RTI from 2375-7P.

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134 Table 6-2. REE partition coefficients. Olivine PlagioclaseClinopyroxene La 0.007 0.19 0.056 Ce 0.006 0.1 0.09 Sm 0.007 0.039 0.445 Y 0.01 0.03 0.9 Yb 0.014 0.067 0.62 Notes: Data from Rollinson, 1993. 4 6 8 10 30 LaCeNdSmEuGdDyYErYbLuSample/Chondrite 2389-5G (Spreading Center A) 2379-2WR (A-B Fault) 2381-11G (B-C Fault) 2378-3G (Spreading Center C) D27-5 (C-D Fault) 2386-5G (Trough D) 2390-9 (RTI) Figure 6-10. Comparison of observed REE trends with modeled REE fractionation trends calculated for 2375-7P from sp reading center B. Thin solid purple lines are modeled REE fractionation tre nds for increments of 10% crystal fractionation. Partition coefficients fo r La, Ce, Sm, Y, and Yb are shown in Table 6-2.

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135 2 4 6 8 10 30 LaCeNdSmEuGdDyYErYbLuSample/Chondrite 2389-5G (Spreading Center A) 2379-2WR (A-B Fault) 2381-11G (B-C Fault) 2378-3G (Spreading Center C) D27-5 (C-D Fault) 2386-5G (Trough D) 2390-9 (RTI) Figure 6-11. Comparison of observed REE trends with modeled REE fractionation trends calculated for D20-15P from the AB fault. Thin solid purple lines are modeled REE fractionation trends for incr ements of 10% crystal fractionation. Partition coefficients for La, Ce, Sm Y, and Yb are shown in Table 6-2. 2 4 6 8 10 30 LaCePrNdSmEuGdTbDyHoYErTmYbLuSample/ChondriteData 26 D23-2 (A-B Fault) A25 D17-9 (A-B Fault) 2384-6 G (A-B Fault) 2388-10 G (A-B Fault) Figure 6-12. Comparison of observed AB fault REE trends with modeled REE fractionation trends calculated for D2 0-15P. Thin solid purple lines are modeled REE fractionation trends for incr ements of 10% crystal fractionation. Partition coefficients for La, Ce, Sm Y, and Yb are shown in Table 6-2.

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136 Compared to the major element mo dels, greater amounts of fractional crystallization are needed to explain the LREE enrichments of the evolved samples from the RTI. Again greater enrichments can be obtained by modeling the magma system as one batch that crystallizes to a greater extent (Figure 6-13). As discussed in Chapter 7, LREE can also be affected by mixing w ith E-MORB compositions. Mixing with EMORB prior to fractional crystallization can al so explain some of the LREE enrichment seen in the more evolved samples. The REE trends of the E-MORBs found at the WRTI are much more enriched in the LREE. Neither 2375-7 nor D20-15 are r easonable parental compositions for the EMORB samples.

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137 4 6 8 10 30 LaCeNdSmEuGdDyYErYbLu5% FC Increments 2389-1 D23-2 2376-8 G 2381-11 G 2378-3 G 2386-5 G 2390-9 99 89.96 79.91 69.87 59.83 49.81 39.79Sample/Chondrite 1 10 100 LaCeNdSmEuGdDyYErYbLu 2389-1 D23-2 2376-8 G 2381-11 G 2378-3 G 2386-5 G 2390-9 99 89.96 79.91 69.87 59.83 49.81 39.79 33.54Sample/Chondrite Figure 6-13. Rayleigh fractio nation model for REE. Models produced by mixing 2384-9 with 6% E-MORB followed by fractiona l crystallization shown for 10% increments. A. Batch fractional crystallization model where the magma chamber is fractionated to a great ex tend without removal of liquid. B. Fractional crystallization model in which the liquid composition is recalculated at 5% increments. Batch crys tallization is able to produce greater enrichment in the REE. A. B.

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138 CHAPTER 7 DISCUSSION Major and trace element data in dicate that the Siqueiros transform domain contains a wide variety of lavas and th at there are at least four gr oups of MORB with distinct chemical compositions. These groups are: 1) normal incompatible element depleted samples recovered from the three intra-transf orm spreading centers, the faults, and trough D; 2) low Na2O samples found at spreading center A; 3) D-MORB found within the A-B transform; 4) E-MORB found at the WRTI. The chemical composition of erupted lavas is generally controlled by 5 f actors. These include the co mposition and mineralogy of the source that melts to form the parental magmas, the depth and extent of mantle melting, the type of melting (e.g. batch, fractional, or polybaric), the am ount of mixing between different magma bodies, and the amount of fr actional crystallization that the magma experiences after melting (Langmuir et al ., 1992; Grove, 2000; Sinton and Detrick, 1992; Perfit and Chadwick, 1988). Fractional Crystallization The majority of the samples from the Siqueiros transform have N-MORB chemical characteristics and were rec overed from all of the intra-transform spreading centers, faults, and even from the RTIs. Most of the chemical variation in the N-MORB can be explained by low-pressure fractional crysta llization as shown by the major element and trace element factional crystallization paths. The majority of the major element data can be explained by 50-55 wt. % fractional crystalliza tion of spinel, olivine, plagioclase, and clinopyroxene from three different primitive N-MORB parental compositions. The

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139 parental compositions used in clude D34-2P (a low sodium parent), 2384-9P, and 2377-7P (a parental composition based on reverse fractional crystalliza tion of a more evolved lava composition). The trace element data are consistent with major element models indicating that the majority of Siqueiros samples formed by 55-60 % fractional crystallization from an N-MORB source si milar in composition to primitive samples recovered within the Siqueiros transform. Th ree of the lavas with primitive compositions were used as potential parental magma co mpositions. D34-2P provided the best overall fit to the observed trace element trends and, although the calculated fractionation trends of D20-15P provide a good f it to much of the observed trace element data, the REE trends of D20-15P indicate that it is even mo re depleted in the light REE than the typical N-MORB and cannot be related to the N-MO RB by fractional crystallization alone. Additionally, the incompatible elements Zr and Y could not be well modeled with the fractionation trends of the th ree parental compositions. The over enrichment of Zr relative to Ti is bette r modeled by extensive fractional cr ystallization of one magma body in which fractionated liquids are not in crementally removed (See Chapter 6). It is clear that, although most of majo r and trace element variation could be explained by fractional crysta llization, there is enough variab ility in the elemental data that more than one parental composition is required in order to explain the entire variation in compositions observed. Sca tter of the major element data around the calculated LLDs indicate that at least 2-3 parental compositions are required or that multiple physical conditions were involved (e .g., slightly different fO2, pressure, water content). The low pressure models of Danyushevsky (2001) do not provide a good fit to the entire range of CaO and Al2O3 data. Better fits to the CaO and Al2O3 data are

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140 obtained by invoking fractional cr ystallization at moderate pressures (~2. 5 kbar) using the model of Langmuir et al. ( 1992). It is also apparent th at many of the more evolved samples are more enriched in incompatible elements (e.g. TiO2, P2O5, and K2O) than predicted by the fractionation of primitive lavas found within Siqueiros. A parental composition similar to 2377-7P is needed in order to explain these “over-enriched” samples by fractional crystallization alone, ye t all primitive lavas recovered within the Siqueiros transform are more depleted in th e most highly incompatible elements than 2377-7P. Magma Mixing and Assimilation Magma mixing can occur between primitive magmas derived from different mantle sources or between primitive and evolved magmas from a similar source. Radiogenic isotopic compositions of indi vidual samples provide the best method to determine whether or not different sources were responsib le for variations in chemical compositions because the isotopic variations cannot be aff ected by melting processes, but reflect longterm differences in the compositions of sour ces. Isotopic measurements (Sr, Nd, and Pb) have been completed on a few of the Siqueir os samples (Sims et al., 2002; Lundstrom et al., 1999), but have confirmed that the E-MORB samples found at the WRTI are isotopically distinct from the N-MORB samp les found within the transform and along the adjacent EPR. This indicates that at leas t two different sources, a typical “depleted MORB source” and a more “enriched” source exist beneath the Siqueiros transform domain. Isotopic analysis of a few DMORB samples showed that they are not significantly different than N-MORB samp les, suggesting that D-MORB and N-MORB sources are similar and that any depletions or enrichments of ra diogenic elements (and other incompatible elements) must have occu rred relatively recently (in geologic terms).

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141 Isotopic variations between the varieties of MORB recovered form the Siqueiros domain limit the amount of mixing between depleted and enriched end members to less than about 5% (see below) however, there is am ple petrologic and chemical evidence that magma crystal mixing has occurred betw een MORB with differe nt major and trace element compositions. The observed scatter in majo r and trace elements might be the result of mixing of evolved and relatively primitive melts (+/crystals). Seismic evidence has shown that a small body of magma (melt lens) overlying a br oad crystal mush zone (crystals + melt) exists beneath most fast spreading ridge s (MacLeod and Yaouancq, 2000; Detrick et al., 1987; Sinton and Detrick, 1992). Little is unders tood about the role the melt lenses play in storage and mixing of MORB. A theory proposed by Pan and Batiza (2003) suggests that the seismically detected shallow melt le nses actually contain highly evolved magma, formed by expelled interstitial melt during cr ystal network compaction. They believe that the composition of many MORB lavas re sult from more primitive magmas that pass through and possibly mix with the evolved melts in the shallow melt lens. This process is supported by evidence from xenocrysts and diverse melt inclusions found in many magmas along the EPR (Pan and Batiza, 2003; Pan and Batiza, 2002; Ridley et al, 2002; Danyushevsky et al., 2003; Kohut and Nielse n, 2003). More evidence for mixing of diverse magma compositions comes from a ga bbroic xenolith rec overed in a young lava from the EPR that contains cumulus anorthite (An > 90) and forsteristic olivine that are out of equilibrium relative to the intersti tial glass between grains and the host rock compositions. Both the plagioclase and olivin e crystals appear to have originated from different melts and the anorth itic phenocrysts lik ely crystallized from a high Ca/Na

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142 primitive melt prior to accumulating with the ol ivine crystals. Anorthitic phenocrysts or xenocrysts are present in many MORB, yet such anorth itic crystals cannot have precipitated from any typical N-MORB melt (K ohut and Nielsen 2003; Ri dley et al., in prep). A number of textural and compositional f eatures in Siqueiros lavas suggest that magma mixing may have occurred during pe trogenesis of the suite. Phenocryst compositions in the Siqueiros samples incl ude calcic plagioclase phenocrysts (An 75-80) that are out of equilibrium with their host glasses and have partially resorbed textures. Large olivine phenocrysts in Siqueiros samples also have partially resorbed rims and Fo contents (Fo = 90) too mafic to have originat ed from their host glasses. In addition, the composition of melt inclusions found in oliv ine phenocrysts from one of the Siqueiros transform picritic basalts are quite divers e and some have compositions believed to reflect assimilation of gabbroic material into hot primitive magma. This assimilation is believed to occur as crystallization begins w ithin the crystal mush zone (Danyushevsky et al., 2003). The presence of phenocrysts (xenocry sts) that are clearl y out of equilibrium with their host rocks and the diverse compositions of melt in clusions indicate mixing of different compositions such as high-MgO me lts and high Ca/Na melts prior to eruption. The role of magma mixing in generating some of the chemical variability observed in the Siqueiros suite can be evaluated using some major element variations. Most plots of individual major element oxides in this suite of lavas show no distin ct inflection points, and as such, provide little or no information regarding the role of magma mixing in the generation of the lavas because with near li near arrays, mixing lines between samples extend along the trend established by differentiation and are consequently

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143 indistinguishable from them. However, prominent inflection points observed in Al2O3, CaO, and CaO/Al2O3 vs. MgO plots (Figure 7-1); c onsequences of the onset of plagioclase crystallization and clinopyroxene crystallization respectively, can provide clues about magma mixing. Lavas with interm ediate compositions that deviate from the predicted LLD trends could be a result of mixing between various evolved liquids with more restricted primitive compositions. Fo r example, mixing a primitive melt, with a composition such as sample 2384-9, with a ferro basaltic melt (like that from spreading center B: 2377-3) or a FeTi basalt (from the RTI: 2390-9) produces mixed melts with intermediate MgO contents and relatively low CaO/Al2O3 values (Figure 7-1). The scatter observed in CaO/Al2O3, the departure of the data from calculated LLDs, and the low CaO contents in some samples are consis tent with mixing of relatively primitive and moderately evolved magmas rather than requiring multiple LLDs produced at higher pressure and/or with different H2O contents. If all of the scatter were a result of evolution along different liquid lines of descent, greater de grees of scatter along other major element liquid lines of descent might be expected. Mixing of more and less evolved magmas is supported by the disequilibri um phase chemical data discussed above. The mixing of magmas cannot however account for the group of low Na2O samples found within spreading center A. These low Na2O samples must either result from partial melting of a low Na source or from greater extents of melting of the mantle beneath spreading center A. Mixing models also provide better fits to some of the observed trends in trace element data than fractional crystallization models. Mixing curves were calculated assuming primitive melt with a composition of 2384-9P mixes with evolved melts with

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144 2377-7P D34-2P 2384-9P 2384-9 2377-3 Mixing Line 2384-9 2390-9 Mixing Line 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95CaO/Al2O3 9 10 11 12 13CaO 12 13 14 15 16 17 18 456789101112Al2O3MgO Figure 7-1. Mixing lines between primitive and evolved sample compositions from the Siqueiros transform. Sample 2384-9 was mixed with a ferrobasalt from spreading center B (2377-3) and a FeTi basalt from the RTI (2390-9). Tick marks on mixing line indicate in crements of 10% mixing.

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145 the compositions of samples 2377-11 or 2390-9 (FeTi basalt from WR TI) (Figure 7-2). Mixing between magmas with such evolved and primitive compositions can explain the observed enrichment in Zr and Y better than fractional crysta llization alone. In these models, the evolved samples must either be derived from a source more enriched in incompatible elements than primitive samples or from melts that have been created by extensive fractional crystal lization. Such highly fracti onated magma bodies may be represented by the melt lens which has been proposed to represent interstitial melt expelled from the extensive mush zone th at underlies ridges (Pan and Batiza, 2003; Natland and Dick, in prep.). Plots of incompatible element ratios (Figur e 7-3) also suggest mixing of melts from different sources may have occurred. The ra tio of two highly incompatible elements is relatively insensitive to the effects of fractiona l crystallization, therefore, relatively large differences in the ratios of incompatible el ements (e.g. Zr/Y; Figure 7-3) are likely to have been inherited from the source region (either because of low extents of melting or because the mantle is heterogeneous). Mixing curves calculated using primitive DMORB samples as one end-member and evol ved samples or E-MORB as the enriched end-members are shown in Figure 7-3. The calculated mixing curves suggest that variations in incompatible element ratios ca n be related to mixing of different magmas that erupted along the Siqueiros transform. Almost the entire range of observed Zr/Y data can be accounted for by mixing of primitive to moderately evolved melts with either E-MORB and/or more evolve d N-MORB magmas represented by lavas found within the Siqueiros transform. Three samples from th e A-B fault have high Zr/Y ratios and fall above the mixing curves. These samples have low Y contents and positive Eu anomalies

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146 40 60 80 100 120 140 160 180 200 0.511.522.53ZrTiO2 16 24 32 40 48 56Y Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection 2384-9P Fractionation Trend 2384-9 2390-9 Mixing Line 2384-9 2377-11 Mixing Line Figure 7-2. Trace element mixing lines between primitive and evolved samples. Mixing increments are 10%. Fractionation tr end for sample 2384-9P is shown for comparison.

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147 2 2.5 3 3.5 4 4.5 5 5.5 406080100120140160180200Zr/YZr Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection 2384-9 2390-1 Mixing Curve 2384-9 2377-11 Mixing Curve Figure 7-3. Calculated mixing curves betw een sample 2384-9 and an evolved sample from spreading center B (2377-11) and an E-MORB from the RTI (2390-1). Blue arrow indicates fractionation trend for 2384-9P.

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148 suggesting that the lavas have either accumula ted plagioclase or assimilated plagioclaserich gabbroic material and thus do no t represent true liquid compositions. D-MORBs and E-MORBs REE element abundances and ratios in lava s from the Siqueiros transform domain exhibit a wide range of valu es not typical for most MORB suites. A nearly continuous trend in Ce/Y and Ce/Yb ratios (measures of LREE to HREE fractionation) can be seen in the Siqueiros sample suite extending from typical N-MORB LREE depleted patterns to patterns almost as depleted as the D-MORB samples found within the A-B fault (Figure 7-4). The overall observed variations in Ce/Y and Ce/Yb ratios cannot be produced by fractional crystallization al one although the variations observed in N-MORB from proximal locations can largely be explained by the effects of crystal fractionation. Even moderate to high percents of crystal fr actionation of olivine, plagioclase, and clinopyroxene only enrich the LREE comp ared to the HREE by approximately 10 relative percent as shown in Figure 7-4. La/Sm ratios (a measure of relative LREE depletion or enrichment) show the extreme LR EE depletions that the samples from the AB fault have compared to other tectonic loca tions within the transform (Figure 7-5). Mixing calculations suggest that the overall range of observed REE patterns (and Ce/Y) could be generated by mixing of depl eted and enriched magmas. Typical NMORB samples could be produced by mixing DMORB with approximately 2-6 % of an E-MORB composition (Figures 7-4 & 7-6) Because the E-MORB are moderately fractionated it is assumed that they have hi gher overall REE abundances than their less evolved parents. Consequentl y, if mixing of parental magm as took place, the amount of E-MORB required could double. In either case, such a small percentage of the enriched melt does not significantly alter the major element composition, but does cause

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149 0.4 0.5 0.6 0.7 0.8 0.9 1 51015202530Ce/Y (Chondrite normalized)Ce (chondrite normalized) FC 0.6 0.8 1 1.2 1.4 00.511.522.53Ce/Y (N-MORB normalized)Ce (N-MORB noramlized) E-MORB MIX FC FC FC FC Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 7-4. Chondrite and N-MORB normalized Ce/Y ratios for Siqueiros transform morphotectonic locations. E-MORB samples from the WRTI are not included. Solid arrow shows fractional crystallization trend for sample 23759. Thin black line shows mixing line between sample 2384-9 (D-MORB) and sample 2390-1 (E-MORB). Blue line shows mixing line between sample 2384-9 (D-MORB) and sample 2377-11 (FeT i basalt). Tick marks indicate increments of 2%.

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150 noticeable enrichment in the incompatible elements which may provide an explanation for the N-MORB that are relatively enriched in TiO2, P2O5, and K2O (Figure 7-7). Once mixed with 6% E-MORB, approximate ly 36% fractional crystallization is required to produce higher overall REE abunda nces similar to those of the N-MORB compositions recovered from the spreading centers (Figure 7-8). Mixing calculations were also done between 2384-9P (D-MORB) and sample 2377-11 (FeTi basalt). Mixing with a more evolved N-MORB compositions will increase the Ce/Y ratios, but does not provide a high enough increase to explain the va riations in Ce/Y ra tios seen between the spreading centers and faults (Figure 7-4). Controls on Spatial Variability in Lava Chemistry A diverse group of samples has been f ound within the Siqueiros transform, however, the spatial distributi on of compositionally distinct samples is very limited and for the most part samples from a common morphotectonic locati on are geochemically very similar (Figure 7-4 and 7-9). E-MORB samples were only recovered from the western ridge transform intersection (WRTI). The extremely incompatible element depleted D-MORB samples were exclusively recovered from th e A-B fault. The rest of the samples recovered from the transform domain range from N-MORB to slightly depleted N-MORB (Figure 7-9). The most evolved N-MORB (ferrobasalts and FeTi basalts) were all recovered from the wester n RTI. Previous dredging of the western section of the transform also recovered a di verse group of samples with similar spacial distributions of E-MORB and N-MORB (Na tland, 1989). Spreading center A has a few normal N-MORB, but for the mo st part contains low-Na2O, low Ce/Y N-MORB samples. The A-B fault contains all of the picritic basalts and picrites recovered within the transform. All of the samples from the AB fault are unusually primitive when compared

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151 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 05101520253035La/Sm (Chondrite Normalized)La (Chondrite Normalized) Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 7-5. Chondrite normalized La/Sm ratio s for Siqueiros tran sform morphotectonic locations. Arrow shows fractional cr ystallization trend. E-MORB samples from the WRTI are not included. A A-B B B-C C RTI D

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152 2 4 6 8 10LaCeNdSmEuGdDyYErYbLuSample/Chondrite D20-15 2375-7 2383-6 2% E-MORB 4% E-MORB 6% E-MORB 8% E-MORB 10% E-MORB Figure 7-6. Calculated mixing between sample D20-15 (D-MORB compositions) and sample 2390-1 (E-MORB composition). Mixing lines (solid purple lines) represent increments of mixing with 2% E-MORB. Approximately 6% mixing with an E-MORB composition is required to pr oduce REE patterns parallel to patterns of N-MORB composition (e.g. 2375-7 and 2383-6).

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153 4681012 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8K2O MgO 0.1 0.2 0.3 0.4 0.5P2O5 Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection 2384-9P Mix 2384-9P and 10% E-MORB Figure 7-7. LLD for 2384-9P after mixing with 10% E-MORB. Mixing with an EMORB composition can explain some of the enrichment in the incompatible major elements

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154 2 4 6 8 10 30 LaCeNdSmEuGdDyYErYbLuSample/Chondrite1% FC (ol) 36% FC (ol + plag) 32% (ol + plag + cpx) D20-15 2375-7 2383-6 6% E-MORB Fractionation Fractionation Fractionation Figure 7-8. Modeled fractional crystallization path of 6% mixing line from figure 7-6. Approximately 36% fractional crysta llization is required to produce REE patterns similar to N-MORB compositions found within the spreading centers (e.g. 2375-7 and 2383-6).

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155 # S # S # S # S # S # S # S # S # S # S # S # S # S % U # S # S % U % U # S # S # S # S # S # S $ T $ T # S # S # S # S # S # S % U % U % U % U # S # S # S # S # S # S # S # S # S # S N% U # S(Ce/Y)n = 0.80 -0.90# S(Ce/Y)n = 0.90 1.0# S(Ce/Y)n = 1.0 -1.4 (N-MORB)$ T(Ce/Y)n = 1.4 -3.84 (E-MORB)(Ce/Y)n = 0.66 0.80 (D-MORB) 800' 8' 815' 815' 830' 8' 845' 845' 1040' 1040' 103' 103' 10330' 10330' 103' 103' Figure 7-9. Location map of E-MORB, N-MORB and D-MORB samples within the Siqueiros transform based on Ce/Y ratios. Ce/Y values normalized to N-MORB.

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156 to the other Siqueiros samples and to samp les collected along the northern EPR (Batiza & Nui, 1992; Pan and Batiza, 2003; Perfit et al., 1994 and in prep; Smith et al., 2001). The majority of samples from the A-B fault ar e D-MORB, however, they were recovered in close proximity to N-MORB samples. Spreading center B is the most densely sampled location. It contains the greatest range in compositions and has the most evolved samples of the 3 spreading centers. Sp reading center B also has lava s with a greater range in Na8.0 and Fe8.0 than the other spreading centers. The B-C and C-D faults, spreading center C, and trough D are not as heavily sampled as th e other locations, but samples from these locations form fairly tight groups on variation diagrams, Na8.0Fe8.0 plots and K2O/TiO2 plots (Figures 5-1 and 5-9). Tectonic Controls on Magmage nesis and Melting Systematics Transform faults are plate boundaries whic h divide active ridge segments and are believed to be places where crus t is neither created nor destr oyed. This assumes that there is no component of extension or spreading in transform domains and that zones of faulting and tectonism dominate the morphologic features. The Siqueiros intra-transform spreading centers are believed to result from a series of plate motion changes occurring about 2.5 Ma, 1.5 Ma, and 0.5 Ma that genera ted extension across the transform. (Pockalny et al., 1997) (Figure 7-10). The formation of the intra-transform spreading centers appears to be the resu lt of tears or propagation even ts initiated near the trace of the transform fault (Pockalny et al., 1997). The tensional environment caused a scissorlike opening of the transform and propagation of the WRTI southwar d (Pockalny et al., 1997). Structural trends within the swath of terrain generate d at spreading center B range from oblique to nearly ridge parallel indicating that the intra-transform spreading centers may begin as leaky transforms, evolving to a transform parallel volcanic ridge as

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157 extension occurs (Pockalny et al., 1997). The recent volcanism within the A-B fault may be a result of current extensi on within the transform. Con tinued extension may lead to more volcanism within the A-B fault and eventually the formation of a new spreading center. Where spreading ridges intersect transform faults (RTI) the thermal regime is believed to be cooler than the ridge due to the juxt aposition of thin, young, hot lithosphere against thicker, older and colder lithosphere. Oceanic crust has been found to be thin proximal to ridge/transform inters ections (Fox et al., 1 981, Stroup and Fox, 1981; Detrick and Purdy, 1980), leading to the idea that the cold edge of lithosphere abutting the end of a segment might affect processe s of basalt generation at the RTI boundary. The processes that lead to the occurren ce of a wider range in magma compositions, magmas of lower temperature, liquid lines of descent that are offset to higher TiO2 and Figure 7-10. Position of “appa rent” Euler poles associated with a counterclockwise change in spreading direction along th e Clipperton and Siqueiros Fracture Zones. t1 = onset of spreading direc tion change resulting in tension along the Siqueiros transform. t2 = New spread ing direction within the Siqueiros Fracture Zone. t3 = extension produced intra-transform spreading centers, a flexural transverse ridge and the abandoned transfor m fault trace with the Siqueiros transform. (Pockalny et al., 1997).

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158 FeO, and magmas with greater incompatible element abundan ces near transform faults has been referred to as the transform fault eff ect (TFE) (Bender et al ., 1984). It has been suggested that the TFE is caused by either gr eater extents of low pressure fractionation (Christie and Sinton, 1981) and/or lower exte nts of melting at the RTI (Bender et al., 1984). It has been proposed that lower extents of melting may lead to preferential sampling of enriched portions of the mantle (E-MORB) (Hanson, 1977; Bender et al., 1984). At the Siqueiros WRTI the intersecti on high is a broad, tongue-like feature with over 300m of relief that spills over the tran sform domain suggesting that the RTI has attempted to propagate southward in the re cent past (Pockalny et al., 1997). As a propagating rift moves into older, thicker, and colder crust and this may lead to increased cooling and crystal fractiona tion (Christie and Sinton, 1981). At the WRTI of the Siqueiros transform, highly fractionated ferrobasalts and a FeTi basalt were recovered along with E-MORB la vas. Relative to other Siqueiros N-MORB samples the RTI ferrobasalts and FeTi basa lt are more enriched overall in REE and incompatible trace elements indicating that they are more evolved and have undergone greater extents of low pressu re fractionation. The RTI N-MORB are also slightly more enriched in incompatible elements compared to other Siqueiros N-MORB. For example, La/Sm (chondrite normalized) ratios of samp les from the WRTI (avg. = 0.72, median = 0.69, not including E-MORB samples) are on av erage greater than the La/Sm (chondrite normalized) ratios of samples from the in tra-transform spreading centers (avg. = 0.60, median = 0.62). It might be expected that all of the in tra-transform lavas would exhibit chemical characteristics indicative of the TFE because lithosphere within the transform is also

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159 older and would be assumed to be colder than lithosphere along the adjacent ridge segments. Instead, lavas within the Siqueir os transform and othe r transforms which exhibit intra-transform volcanism have been found to be more porphyritic, less evolved, and have lower concentrations of incompatible trace elements compared lavas from adjacent ridge segments (Wendt et al., 1999, Perfit et al., 1996). Constraints on Melting –Na-Fe Systematics Mid-ocean ridge basalts (MORB) are produced by decompression melting of the upper mantle in response to plate separation. Low pressure crystallization results in major element variations that form rather smooth trends on plots of oxide abundances as a function of MgO. As magma cools MgO, wh ich is compatible in olivine, decreases during low-pressure crystallization (L angmuir et al., 1992). The cooling and crystallization of olivine produces changes in the concentrations of all the elements. These chemical changes must be corrected fo r in order to see trends caused by more complex fractionation processes, processes of melt generation and segregation, or source heterogeneity (Langmuir et al., 1992). The effects of fractional crystallization can be corrected for by normalizing the major elemen t concentrations to a constant MgO. The Siqueiros sample Na2O and FeO contents were nor malized to a MgO content of 8.0 wt. % in order to observe any local variability in composition due to processes other than fractional crystallization (A ppendix F). The Siqueiros transform Na8.0 and Fe8.0 data group in the center of the global Na8.0-Fe8.0 field (Figure 7-11). For the most part the samples from comm on morphotectonic locations gr oup together, except for samples from the A-B fault and spreading center B, which have a wide range in Na8.0 and Fe8.0 (Figure 7-12).

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160 Globally, Na8.0 and Fe8.0 values have been found to vary in basalts from normal ridges, basalts from back-arc basins, and for basalts from ridge segments that have been influenced by certain hotspots. Normal ridge Na8.0 and Fe8.0 values in MORB correlate strongly with axial depth assuming ma ntle major element compositions are approximately the same worldwide (Langmuir et al., 1992). Deeper axial depths have been found to correlate with higher Na8.0 and lower Fe8.0 values. The global Na8.0-Fe8.0 trend has been attributed to variations in man tle temperature. Axial depth variations are a response to variations in mantle temperat ure beneath the ocean ridge depth. Hotter mantle has undergone a higher extent of melting and correlates with a more inflated axial ridge (Grove, 2000). This results in a global trend that has a negative correlation between Na8.0 and Fe8.0. Na2O behaves as a moderately incompatible element; therefore, shallow melt regimes with a short melt column (low % of melt) will have high Na2O, whereas deeper melts with a longer melt column (high % of melt) will have lower Na2O. Mantle temperatures have been found to have the opposite effect on FeO concentrations because deeper melt columns have higher average pre ssures, which have been found to correlate with higher FeO. Variation in the temperature of the mantle is the only process that has been found to produce a nega tive correlation between Na8.0 and Fe8.0 (Klein and Langmuir, 1987). In addition to these global va riations, finer “local” variations that are opposite the global trend have been observed that indicate distinct ch emical signatures for individual sections of the ocean ridge system. The regional data for slow-spreading ridges form trends that parallel the global tr end, but each local region has trends that are oblique to the global trend. Th ese trends appear to reflect processes that occur beneath individual ridge segments (Langmuir et al., 1992).

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161 Local variability of data from the East P acific Rise (EPR) is much different than that of slow-spreading ridges (Langmuir et al., 1992) because the EPR shows much less depth variation than slow spreading ri dges like the Mid-Atlantic Ridge. The Na8.0 and Fe8.0 data sets for the EPR parallel the gl obal vector and the range for individual segments are almost as large as the global range. The average EPR data plot in the middle of the global field. The variability obser ved in EPR lavas occurs over distances as small as 50 km. Examination of other major elements suggest that the scatter of the Na8.0 and Fe8.0 data may be due to two components of lo cal variability, one within the ‘normal’ MORB (N-MORB, with low K2O/TiO2), and one between N-MORB and ‘transitional’ MORB (T-MORB, with higher K contents). Local variations emerge when the N-MORB is considered alone. However, the EPR does not have a striking Na8.0-Fe8.0 negative correlation because N-MORB exhibit little variation in Na8.0. The variation in Na8.0 comes from mixing N-MORB with T-MORB or E-MORB, which have low FeO and high Na2O. When compared to the global field fo r normal ridge segments the Siqueiros samples parallel the global vector, and there is significant variability in the Na8.0 (Figure 7-11). Although there is variability in Fe8.0 it is not as great. Unlike other normal ridge segments the Na8.0 and Fe8.0 does not show a correlation with axial depth (Figure 7-17). Brodholt and Batiza (1989) found that the globa l trends are strongly defined by samples from very shallow and very deep ridges and that the Fe8.0 trend is particularly weak for normal depths (1500-4000m). The Siqueiros samples were only collected for depths ranging from 1990m to 3909m and w ould not be expected to show the entire range in the global Na8.0-Fe8.0 variations. The general trend of the Siqueiros data sugg ests that there

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162 1.5 2 2.5 3 3.5 789101112Na8.0Fe8.0 Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 7-11. Siqueiros Na8.0 and Fe8.0 data compared with global field for normal ridge segments (Langmuir et al., 1992).

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163 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 77.588.599.510Na8.0Fe8.0 Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 7-12. Na8.0 vs. Fe8.0. The RTI samples form a group with high Na8.0, low Fe8.0 and samples from spreading center A group in the low Na8.0, high Fe8.0 region. All iron is calculated as FeO.

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164 are different mantle temperatur es controlling the Siqueiros Na8.0-Fe8.0; however, the greater variability in Na8.0 may result from local trends in the Na8.0 and Fe8.0 that are opposite to the observed global trends. The local trend in the Siqueiros data ma y result from varying source compositions. In other EPR lavas, Na8.0 abundances have been found to correlate with higher K2O/TiO2 contents. Locations that exhibit a wide range in Na8.0 abundances also exhibit ranges in K2O/TiO2 and Ce/Y contents (Figure 7-13 and 7-14). A subset of RTI samples are characterized by higher Na8.0 and lower Fe8.0, correlate with the high K2O/TiO2 E-MORB samples from the RTI. The low Na2O, high FeO samples from spreading center A correlate with low Ce/Y (N-MORB normalized) suggesting that the Na8.0 variations may result from mixing between depleted and enrich ed sources (Figure 7-15). As a whole the Siqueiros Na8.0 data does not correlate with Ce/Y (N-MORB normalized) or K2O/TiO2 (Figures 7-14 and 7-16). Another important consideration is that the global data set was produced from regional averages of lava chemistry and depth (Klein and Langmuir, 1987). Using averages results in an average of the depth a nd pressure of melting for a particular region. Prior to averaging of the data melts may ex ist that result from the eruption of lavas originating from different depths or pre ssures within the melting column. If one considers shallow, high percent melts and d eep, low percent melts in MOR mantle it is possible to generate more variability in Na8.0, Fe8.0 by producing high Fe8.0, high Na8.0 deep melts and low Fe8.0, low Na8.0 shallow melts (Figure 7-18). The lack of correlation between Na8.0-Fe8.0 systematics with depth, K2O/TiO2, or Ce/Y indicates that Na8.0 and

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165 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 88.599.51010.511Na8.0Fe8.0 0 0.05 0.1 0.15 0.2 0.25 0.3K2O/TiO2 Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 7-13. Na8.0 vs. Fe8.0 and K2O/TiO2. The RTI samples form a group with high Na8.0, low Fe8.0 and samples from spreading center A group in the low Na8.0, high Fe8.0 region. All iron is calculated as FeO.

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166 0.6 0.8 1 1.2 1.4 2.22.32.42.52.62.72.82.93 a a-b b b-c c c-d d rtiCe/Y (N-MORB Normalized)Na8.0 Figure 7-14. Ce/Y ratios vs. Na8.0 values for all Siqueiros transform samples. 0.8 0.85 0.9 0.95 1 1.05 1.1 1.15 1.2 2.22.32.42.52.62.7 y = -0.94901 + 0.78066x R= 0.83978 Ce/Y (N-MORB normalized)Na8.0 Figure 7-15. Ce/Y ratios vs. Na8.0 values for samples from spreading center A. A best fit line shows that there is a good correlation between LREE enrichments and higher Na2O in samples from this spreading center.

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167 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # ## # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #K2O/TiO2#0.011 0.038#0.038 0.065#0.065 0.166#0.166 0.332Increasing Fe8.0Increasing Na8.0 Figure 7-16. K2O/TiO2 ratios of Siqueiros samples compared with their Na8.0, Fe8.0 data. ( X( X( X ( X ( X ( X ( X( X( X( X ( X ( X( X ( X( X( X ( X( X ( X( X( X( X( X( X( X( X ( X ( X( X( X( X( X ( X ( X( X( X( X ( X ( X( X( X( X ( X( X( X( X( X ( X( X( X ( X( X( X ( X( X ( X( X( X( X( X( X( X( X( X( X ( X ( X( X( X ( X ( X ( X( X( X( X ( X( X ( X( X( X ( X( X( X( X( X( X ( X ( X ( X( X( X( X ( X( X( X ( X( X( X( X ( X( X( X( X( X( X ( X( X( X( X( X ( X ( X( X( X( X ( X( X( X ( X( X( X ( X ( X( X( X( X( X( X( X( X( X( X( X( X( X( X( X( X( X( X( X( X( X( X( X( X( X( X ( X( X( X( X( X( X ( X( X( X( X( X( X( X( X( X( X ( X( X( X ( X( X ( X( X( X( X( X( X( X ( X( X( X( X ( X( X( X( X( X( X( X( X( X( X ( X( X( X ( X( X( X( X( X ( X ( X( X( X( X ( X( X( X ( X( X ( X ( X( X( X ( X( X ( X ( X( X( XDEPTH ( m ) ( X2023 2352( X2353 2800( X2801 3037( X3038 3206( X3207 3909Increasing Fe8.0Increasing Na8.0 Figure 7-17. Fe8.0 versus Na8.0 and axial depth.

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168 shallowmelts, low%melt deepmelts, low%melt shallowmelts, high%melt deepmelts, high%melt Figure 7-18. Variations in Na8.0 and Fe8.0 systematics due to vari able depths and extents of melting. Fe8.0 systematics result from two components of local variabili ty, one due to variable mixing with enriched sources and the other due to variable depths and extents of melting. Models for Volcanism in Transform Domains. Perfit and others (1996) hypot hesize that the high-magnesian samples are only found within the A-B fault because of their grea ter density. Since the A-B fault is deeper than other areas, it was suggest ed that the transform might ta p the high-MgO lavas stored in a magma chamber that are too dense to erupt at shallower locations. In order to evaluate this, the densities of each Siqueiros sample were calculated (using the method of Lange & Carmichael, 1987) in order to see if density is related to sample depth of recovery. The density of the samples does not correlate with dept h and a best-fit line

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169 actually shows an extremely poor inverse co rrelation with denser samples coming from shallower regions (Figure 7-19). A comparis on of MgO contents with depth was also made and although the high MgO lavas are found at the deepest locations, there does not seem to be any other correlation between MgO content and depth for the Siqueiros sample suite (Figure 7-20). When the olivin e phenocrysts contained in the picritic and olivine rich basalts are taken into account, a large differenc e in sample density can be seen. The picritic basalts have been found to contain up to 20 modal% olivine. The addition of 5-20 modal% olivine phenocrysts to th e liquids of the picritic and olivine rich samples greatly increases their sample dens ity (Figure 7-21). Th e picritic and olivine basalts were recovered in the deepest sample locations and an addition of only 5% olivine phenocrysts to these samples shows that a co rrelation can be made between density and sample depth (Figure 7-22). Another theory regarding the eruption of high-MgO la vas only within transform domains is that the relatively primitive and por phyritic basalts result from rapid transport of magmas to the surface w ithout extensive cooling and fr actionation in crustal magma chambers (Hekinian et al., 1995; Wendt et al ., 1999). Hekinian and other (1995) propose that the ascent of magma through narrow fissures or dykes in the fast cooling environment of the transform would increase th e magma’s viscosity as well as the rate of crystal nucleation and th erefore prevent cr ystal settling, leading to the extrusion of highly porphyritic lavas. The high-MgO samples were only recovered in the A-B fault located away from areas of organized spreading were it is unlikely that a large magma chamber exists. The narrow range in lava MgO content in the A-B fault is consistent with limited mixing and fractional crystallization.

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170 Geophysical evidence indicates that sh allow melt lenses may contain > 70% crystals, whereas dikes and lavas generally have < 10% (Hussenoede r et al., 1996). Yet dikes and lavas erupted along the EPR have M g#s too high to have formed from > 70% crystallization of mantle-derived liquids. Many researchers believe that the lavas instead have a deeper source such as a sill near the Moho that supplies magmas erupted at the surface and that the shallow melt lenses are unrelated to lavas erupted at the surface (Kelemen et al., 1997; Natland & Dick, in prep; Pan & Batiza, 2003). Studies of ophiolites have recently led many to the conc lusion that lower gabbros and perhaps much of the oceanic crust forms from sills (Kelemen et al., 1997). Calculated liquids for gabbro sills in the Oman mantle transition zone are id entical to the composition of sheeted dikes. Also, PmS refractions near the Moho within 30 km of the EPR may indicate the presence of sills (Kelemen et al., 1997). Recent co mpliance techniques discussed in Kelemen and others (1997) and studies of ophiolites suppor t the existence of lower crustal sills (Natland & Dick, in prep). Compliance techni ques indicate that in addition to an upper melt lens at ~1.5 km below many ridges, a lower me lt lens exists at the base of the crust. Natland & Dick (in prep.) propose that the lo wer melt lens corresponds to a zone where picritic melts are neutrally buoyant causing oliv ine-rich magmas to laterally intrude the lower crust. Natland & Dick (i n prep.) also propose that bot h plagioclase-rich & picrite basalts can be erupted due to flowage differen tiation of crystals in the dikes or sills. Garrett Transform Models Insights into the petrogene sis within Siqueiros can be gained from published studies of other intra-transform domains. The Garrett transform on the SEPR is one of the only well studied transforms exhibiting intr a-transform volcanism. Within the Garrett transform the majority of lavas were found to be more porphyritic and less evolved than

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171 2.64 2.66 2.68 2.7 2.72 2.74 20002500300035004000 y = 2.6935 2.9867e-06x R= 0.1323 Density (kg/m3)Depth (m) Figure 7-19. Sample density versus recovery depth for Siqueiros samples. Best fit linear line shows a very poor reverse correlat ion between sample density and depth. 5 6 7 8 9 10 11 12 13 20002500300035004000MgO (wt. %)Depth (m) Spreading Center A A-B Fault Spreading Center B B-C Fault Spreading Center C C-D Fault Trough D Ridge Transform Intersection Figure 7-20. Sample MgO content versus recovery depth for Siqueiros samples.

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172 2.7 2.8 2.9 2.7 2.8 2.9 150020002500300035004000 Density of Liquid 5% Olivine 10% Olivine 15% Olivine 20% OlivineDensity (kg/m3)De p th ( m ) Figure 7-21. Density of samples vs. depth w ith addition of olivine phenocrysts. Between 5-20% olivine phenocrysts were include d in the calculation of the sample density. 2.72.7 20002500300035004000 y = 2.6672 + 8.4594e-06x R= 0.27036 Density (kg/m3)Depth (m) Figure 7-22. Density vs. depth of Siqueiros samples with 5 modal % olivine added to picritic and olivine rich basalts.

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173 those along the adjacent South East Pacific Rise (SEPR) (Hekinian et al., 1995). The lavas are more depleted having lower concentrat ions of incompatible trace elements than normal MORB (Wendt et al., 1999). Sr, Nd, and Pb isotope compositions overlap those in the depleted end of the Pacific mid-ocean ridge basalts, but extend to less radiogenic Sr and Pb isotopes and more radiogenic Nd isot ope values (Wendt et al., 1999). The Garrett transform also contains fe rrobasalts, which fall along elemental trends suggesting extensive fractional crystallization. Two models have been proposed to explain the existence of highly depleted basalts and the absence of enriched basalts in the Garrett transform domain. The first model proposes that enriched melts are generated in the transform, but because of their small volume and the cooler thermal regime they freeze in the lithosphere before extrusion (Hekinian et al., 1995). With continued and more extensive melting the ascent of magmas leads to reheating of the lithosphere allowing depleted, primitive melts to be extruded. Small magma chambers may be produced where only small extents of fractional crystallization and mixing take pl ace. Crystallization and accumulation in the magmatic reservoir may later lead to the ex trusion of more evolved and aphyric lavas (Hekinian et al., 1995). Wendt and others (1999) poi nt out that this model requires a unique process that causes enriched and deplet ed melts from the same source to remain separate beneath transforms. Wendt and ot hers (1999) instead proposed a second model in which the D-MORBs are believed to re sult from the melting of a two-component mantle beneath a transform. The model of Wendt and others (1999) requires that the upper mantle material currently melting bene ath the Garrett transform has lost the enriched, easily melted component during prev ious partial melting beneath the SEPR, yet

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174 remains sufficiently fertile to undergo decompression melting during lithospheric extension with the Garrett transform. Wendt and others propose that beneath the EPR deeper melts undergo less melting and remain fertile enough to melt underneath the Garrett transform where melting is occurring de eper due to the colder thermal regime. Such a model would require that that the me lting beneath the EPR axis was very minimal because even a small amount of melt results in a refractory mantle that is very difficult to melt and yields melts very different from N-MORBs (Falloon et al ., 1997). The model also implies that the Garrett tr ansform is hot enough to melt th e residual material that was not melted beneath the EPR axis suggesting the isotherms are higher than under normal ridges. Siqueiros Transform Models The Siqueiros transform differs from the Ga rrett transform in that the majority of samples recovered within Siqueiros, although depleted in K/Ti, have REE patterns and isotope compositions similar to N-MORBs (L undstrom et al., 1999) and unlike Garrett, E-MORBs have been recovered at the WRTI D-MORB samples were recovered within the Siqueiros transform, but the highly depl eted LREE samples are only found within the A-B fault, whereas, D-MORB were recovere d from the faults zones and the intratransform ridges within the Garrett transform. Samples from the ot her spreading centers and faults have Ce/Y ratios that range from N-MORB to slightly depleted N-MORB (Figure 7-9). U-series disequilibria measurements of Si queiros lavas revealed that there is an inverse correlation between 230Th excess and 226Ra excess (Lundstrom et al., 1999; Sims et al., 2002). The 226Ra and 230Th excesses are found to vary with composition. 226Ra excesses are found in the D-MORBs and are positively correlated with Mg# (Mg/(Mg +

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175 Fe)). Lundstrom and others (1999) explai ned the inverse correlation as a result of heterogeneous source compositions and suggest that N-MORBs, which have intermediate 226Ra excesses, result from mixing of a D-MORB source with 5-10% E-MORB. Lundstrom and others (1999) concluded that these mixing trends indicate that intratransform lavas undergo melting processes similar to those beneath the ridge. Recently U-series disequilibria coupled with 87Sr/86Sr data has been used to argue for melting processes rather than source he terogeneity as the dominant c ontrol on variations in Th/U (Sims et al., 1999). Incompatible enriched me lts which have only been found at the RTIs in Garrett and Siqueiros have high Th/U and 230Th excesses, while the most depleted incompatible element melts are found at lea ky transform faults and are characterized by low Th/U and low 230Th excesses. Melts intermediate in Th/U and 230Th excesses were found at the intra-transform sp reading centers in Garrett a nd Siqueiros. The cause of variation could be either di fferent long-lived sources or melting processes. Recent studies have shown that a significant am ount of trace element and U-series nuclides variability can occur as a result of melti ng processes (Speigelman and Kelemen, 2002). The Sr, Nd, Pb, and Hf isotopic com positions of the 9-10N EPR N-MORB and Siqueiros D-MORB samples measured by Sims et al. 1999, were found to be relatively homogeneous, but the Siqueiros E-MORB is is otopically enriched. Based on the isotopic similarities of the D-MORB and N-MORB sa mples it was concluded that a melting process such as progressive source depletion during polyba ric melting is the dominant control on variations in Th/U. However, isotopically depleted source components have been found at the Lamont Seamounts and suggest that an isotopically depleted component does exist in the 9N area (Forna ri et al., 1988; Tepley et al., 2004). The

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176 Siqueiros REE patterns indica te that D-MORB only require 4-6% E-MORB to produce REE patterns similar to N-MORB. When 4-6% E-MORB is mixed with the D-MORB samples from Siqueiros the 87Sr/86Sr ratios are within the range of Siqueiros N-MORB 87Sr/86Sr ratios reported by Sims and others (2002), indicating th at the U-series disequilibria could still be a result of source heterogeneity. Proposed Model The Siqueiros transform petrogenetic model must account for the following observations: 1) Like the Garrett transform, the samples from the Siqueiros transform are more primitive and porphyritic than those recovered along the EPR. 2) Major element variations indicate that fractional crystalliza tion is occurring beneat h the transform. The greater range in MgO contents found at the spreading centers indicates that they have undergone greater amounts of fractional crysta llization than samples recovered from the fault zones. 3) Radiogenic is otope analysis confirms that th ere are at least two different sources beneath the transform (E-MORB a nd N-MORB). 4) REE patterns of D-MORB samples indicate that they cannot be relate d to N-MORB by fractional crystallization, but require mixing with 4-6% E-MORB to produce REE patterns similar to N-MORB samples. 5) Evidence for mixing is seen in phenocrysts/xenocryst textures and compositions. Mixing model between evolved and primitive samples provide better fits to major and trace element data. 6) Sa mples from similar morphotectonic locations group together on Ce/Y and Na8.0, Fe8.0 diagrams, but Ce/Y ratios from different locations cannot be produced by fractional crystalli zation. Ce/Y ratios are can be best explained by variable extents of mixing with an E-MORB source. Na8.0, Fe8.0 data cannot be easily explained by mixing different sources or by variability in the depth and extents

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177 of melting. Instead, the Na8.0-Fe8.0 variability appears to result from a combination of source variability and vari ability in extents and depths of melting. The above observations can be fit by a petrogenetic model in which lava compositions are controlled by the presence, si ze, and depth of melt lenses located within the transform (Figure 7-23). The eruption of N-MORB and evolved samples from the spreading centers opposed to the D-MORB and pr imitive basalts erupted within the faults can be explained by the existence of melt le nses located beneath the spreading centers that mix depleted and enriched sources and fr actional crystallize magm a prior to eruption. The fairly narrow range of compositions found at individual tectonic locations suggests that some homogenization does occur within melt lenses located beneath the spreading centers. The melt lenses are probably truncated by the fa ult zones resulting in the eruption of more depleted, primitive basalts with in the faults. The more depleted nature of the samples from the A-B fault suggests that sources may be mixed with lower percents E-MORB or none at all. The na rrow range in MgO contents and lack of ferrobasalts recovered from the fa ults indicates that limited fractional crystallization is occurring beneath the faults (i.e. magmas may not be mixed in a shallow level magma chamber). The higher density of the picriti c samples may have caused them to erupt within the A-B fault because it is deep enough to tap a lower melt lens or because it is not filtered through an upper melt lens. The WRTI may tap the very edge of the EPR melt lens where highly evolved samples can be erupted along with E-MORB samples due greater amounts of fractiona l crystallization in a colder thermal regime. Similar models (Natland and Dick, 1996 and in prep.; Kelemen et al., 1997, Boudier et al., 1996) have been proposed in which sills located at various depths feed

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178 erupted lavas (Figure 7-23). These include the upper melt lens located beneath the sheeted dikes and a lower melt lens located at the base of the crust where picritic basalts are neutrally buoyant, but may also include small melt lenses located at variable depths in between. Larger sills where magmas mix to a greater extend might be located beneath the more homogenized and fractionated spread ing centers, while the deeper faults tap smaller sills of variable depths that have not undergone as much mixing and fractionation. Such a model can be used to explain the Na8.0 and Fe8.0 data which suggests eruptions of variable depths and melt percents and al so to explain the variability in sample composition found at small spatial intervals. Mixing of compositions may also occur as lavas are channeled to the surface. Numerical models suggest that large variations in prim ary magma composition can be caused by channelized melt transport thr ough the mantle (Spiegelman and Kelemen, 2003). The centers of channels can contain en riched melts from depth, while the edges of the channels transport highly depleted melts extracted from the inter-channel regions at shallower levels (Spiegelman & Keleman, 2003). If channelized melt transport is occurring in Siqueiros transform, it would help explain the wide variations in Na8.0, Fe8.0 data found at the spreading cen ter B and the A-B fault. Within the melt column deep melts will carry one Na8.0, Fe8.0 signature, while melts equili brated at shallow depths along the outside of the column ma y carry a completely different Na8.0, Fe8.0 signature. Models for channelized melt transport, me lt inclusions, and a northitic plagioclase phenocrysts indicate that there is a wide range of com positions which are rarely erupted, but most likely are a component of MORB parental magmas.

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179 Figure 7-23. Magma transport within the Siqueiros transform (modified from Natland & Dick, in prep.).

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180 CHAPTER 8 CONCLUSIONS Based on major and trace element models, mixing models, and th e spatial locations of samples the following conclusions can be made about the Siqueiros transform. 1. Elemental fractionation trends similar to that seen in samples recovered from the EPR suggest that similar magmatic proce sses are occurring wi thin the transform domain. The majority of the Siqueiros major and trace element variations can be explained by fractional crysta llization of parental compositions similar to the high MgO basalts recovered form the A-B fault, with the most evolved samples requiring 50-60% fractional crystallization of spinel + olivine plagioclase clinopyroxene. Samples from a common mo rphotectonic locati on can generally be related by fractional crystallization and the spreading centers show the greatest range in MgO, appearing to have undergone more fractionation than those from the faults. The majority of samples recovere d from the spreading centers are also NMORB. These characteristics suggest that fairly long-lived magma chambers or melt lenses capable of fractionating mixing magma bodies exist beneath the spreading centers. This is in agreemen t with tectonic models suggesting that “normal” crustal accretion has been occurr ing along the spreading centers for a few million years. 2. Although most of the chemical variability in the lavas can be explained by fractionation, variations in the major a nd trace element data are great enough to require at least 2-3 different parental compositions. The A-B fault contains both DMORB and N-MORB and E-MORB are presen t at the nearby RTI. The close proximity of these chemical types suggest s that mantle heterogeneities exist on a very small scale. 3. Well evolved melt lenses or long-lived sills probably do not exist beneath the faults. The more primitive and porphyritic nature of samples from the A-B fault, along with the occurrence of va riably depleted samples may result from the lack of well developed melt lenses with the fault z ones. Here the lavas may be channeled directly to the surface with limited fractiona tion in sills or channels. The result is the eruption of more primitive, porphyritic samp les, due to the lack of a melt lens in which crystals are removed and melts are filtered. 4. Mixing of some magmas prior to erupti on within the transform is supported by presence of the phenocrysts/xenocrysts th at have textures a nd compositions that indicate that they are out of e quilibrium with their host rocks.

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181 5. Incompatible element ratios, CaO, Al2O3, Zr, and Y trends can be best fit by mixing models between primitive and evolved compositions. Evolved compositions may be mixed with primitiv e magmas in the upper melt lens, which has been recently proposed to contain highly evolved interstitial melt expelled during crystal network compaction. 6. Radiogenic isotopic analysis of NMORB and E-MORB, REE patterns of DMORB, Ce/Y (N-MORB normalized) ratios, and Na8.0-Fe8.0 data all suggest variable mixing between enri ched and depleted sources. Melt lenses beneath the spreading centers may mix larg er volumes of melt result ing in the eruption of only N-MORB, but variable extents of enri ched material may be mixed at each spreading center resulting in variable Ce/Y and Na8.0-Fe8.0 that cannot be explained by fractional crystallization. The faults between spreading centers, which are not believed to have well developed melt le nses, are not able to thoroughly mix depleted and enriched components and al so may undergo much smaller extents of melting resulting in less enrich ed material being tapped. 7. The crystal mush zones beneath each spr eading center may be of variable sizes resulting in different extents of melting. Channels and sill s that erupt w ithin faults or feed melt lenses may be of variable si zes and located at different depths. The variability in extents and dept hs of melting can explain the Na8.0 and Fe8.0 variations that cannot be e xplained by source variations. Fractional crystallization at moderate pressures (~2-8 Kb)are also lik ely to play a role in the major element variations.. 8. The petrogenesis of the samples within Siqueiros may be controlled by the presence, size, and depth of melt lenses loca ted within the transform. Melt lenses are believed to be located beneath the sp reading centers which exhibit fractional crystallization trends and N-MORB compositions. More depleted and primitive samples found within the A-B fault may result from the lack of a shallow melt lens capable of mixing enriched and depleted components.

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182 APPENDIX A NORMALIZATION OF CAME CA MICROPROBE DATA Graphical comparison of the Cameca el ectron microprobe data and the other microprobe and DCP data showed that there appe ars to be systematic analytical biases in the MgO and P2O5 contents of the data sets. Th e MgO contents obtained from the Cameca electron microprobe are consistently hi gher than the MgO contents of the ARL and JEOL electron microprobes and the P2O5 contents of the Cameca electron microprobe data are consistently lower than the P2O5 contents of the ARL and JEOL electron microprobes. The Cameca electron microprobe data was normalized to be consistent with the ARL and JEOL electron microprobe data. Normalizations were made to the Cameca electron microprobe MgO and P2O5 concentrations by plotting the Came ca electron microprobe MgO and P2O5 values versus the other microprobe MgO and P2O5 contents for replicate samples (Figure A-1). A best fit line was matched to the data. For a perfect f it, the best fit line w ould have a slope of 1. In order to remove the analytical offset, the MgO and P2O5 contents of the Cameca electron microprobe data were shifted to make a best fit line with a slope equal to 1 (Figure A-2). The following equation was used to correct the MgO contents: MgOcorrected = (MgOCameca + 0.026281) / 0.82046 The following equation was used to correct the P2O5 contents: P2O5corrected = (P2O5Cameca –0.44722) / 0.90029

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183 5 6 7 8 9 10 567891011 y = 0.44722 + 0.90029x R2= 0.97776 Cameca SX50 microprobe MgOARL and JEOL microprobe MgO 0 0.05 0.1 0.15 0.2 0.25 00.050.10.150.20.250.30.35 y = -0.026281 + 0.82046x R2= 0.94503 Cameca SX50 microprobe P2O5ARL and JEOL microprobe P2O5 A. B. Figure A-1. Cameca microprobe data versus ARL and JEOL microprobe data. A. Best fit line showing the offset between th e Cameca microprobe MgO contents and the ARL and JEOL microprobe MgO conten ts. B. Best fit line showing the offset between the Cameca microprobe P2O5 contents and the ARL and JEOL microprobe P2O5 contents. The equations for the best fit lines ar e displayed at top of graphs along with the R2 value.

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184 5 6 7 8 9 10 11 567891011 y = 5.4515e-06 + 1x R2= 0.97776 Adjusted Cameca SX50 microprobe MgOARL and JEOL microprobe MgO 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 00.050.10.150.20.250.30.35 y = 4.5596e-07 + 1x R2= 0.94503 Adjusted Cameca SX50 P2O5 ARL and JEOL microprobe P2O5A. B. Figure A-2. Normalized Cameca microprobe data. A. Normalized Cameca microprobe MgO contents versus ARL and JEOL microprobe MgO contents. B. Normalized Cameca microprobe P2O5 contents versus ARL and JEOL microprobe P2O5 contents. The MgO and P2O5 contents were shifted to fit a slope of 1. The equation for the best fit li ne is displayed at the top of graphs along with the R2 value.

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APPENDIX B OLIVINE, PLAGIOCLASE AND SPINEL MICROPROBE ANALYSIS

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186Table B-1. Microprobe analys is of olivine phenocrysts in the Siqueiros samples. Sample # Description Mg* SiO2FeOTMnOMgO CaOTotalSi Ti Fe MnMg Ca CrOFo Fa 2375-9-ol1 ctr. sm. euh. 0.6 39.8 15.080.12 45.050.38 100.431.00 0.00 0.320.001.68 0.010 4 84.19 15.81 50% outward 0.6 39.7 14.790.17 44.540.38 99.581.00 0.00 0.310.00 1.67 0.010 4 84.29 15.71 rim 0.6 39.5514.920.2 44.4 0.39 99.461.00 0.00 0.320.00 1.67 0.010 4 84.14 15.86 2376-3-ol1 ctr. sm. euh. 0.62 40.1413.520.16 45.790.28 99.891.00 0.00 0.280.00 1.70 0.010 4 85.79 14.21 2376-8-ol1 ctr. sm. euh. 0.61 40.1314.090.16 45.810.26 100.451.00 0.00 0.290.00 1.70 0.010 4 85.28 14.72 2377-4-ol1 ctr. sm. euh 0.59 39.7515.330.19 44.560.31 100.141.00 0.00 0.320.00 1.67 0.010 4 83.82 16.18 2377-4-ol2 lge. anh. 0.59 39.5116.910.21 43.460.23 100.321.00 0.00 0.360.00 1.64 0.010 4 82.08 17.92 rim 0.59 39.7316.980.16 43.450.24 100.561.00 0.00 0.360.00 1.63 0.010 4 82.02 17.98 2377-11-ol1 ctr. lge. euh. 0.56 39.5917.880.22 42.990.27 100.951.00 0.00 0.380.00 1.62 0.010 4 81.08 18.92 2378-7-ol1 ctr. sm. euh. 0.6 39.8415.090.19 45.2 0.33 100.650.99 0.00 0.320.00 1.68 0.010 4 84.22 15.78 2378-7-ol2 ctr. sm. euh. 0.6 39.7514.840.16 44.670.33 99.751.00 0.00 0.310.00 1.68 0.010 4 84.29 15.71 2380-4-ol1 ctr. sm. euh. 0.59 39.9215.430.21 44.730.25 100.541.00 0.00 0.320.00 1.67 0.010 4 83.78 16.22 2380-11-ol1 ctr. sm. euh. 0.61 39.6816.550.2 43.930.25 100.611.00 0.00 0.350.00 1.65 0.010 4 82.55 17.45 2381-11-0l1 ctr. sm. euh. 0.61 39.4114.190.13 45.080.37 99.181.00 0.00 0.300.00 1.70 0.010 4 84.99 15.01 2381-11-ol2 ctr. sm. euh. 0.61 40.1112.530.14 46.820.34 99.941.00 0.00 0.260.00 1.73 0.010 4 86.94 13.06 2381-11-ol1a ctr. sm. euh. 0.61 39.9214.240.18 45.480.37 100.191.00 0.00 0.300.00 1.69 0.010 4 85.06 14.94 2382-7-0l1 glomero. w. cpx+plag 0.58 40.0316.470.2 44.020.26 100.981.00 0.00 0.34 0.00 1.64 0.010 4 82.65 17.35 2383-2-ol1 ctr. sm. euh 0.59 39.6315.380.26 44.7 0.28 100.251.00 0.00 0.320.00 1.67 0.010 4 83.82 16.18 2383-6-ol1 ctr. med. euh. 0.67 40.6 12.550.13 47.250.26 100.791.00 0.00 0.260.00 1.73 0.010 4 87.03 12.97 rim 0.61 40.5512.510.15 47.160.33 100.71.00 0.00 0.260.00 1.73 0.010 4 87.04 12.96 2384-1-ol1 ctr. lge. anh. 0.71 41.2210.090.14 48.530.28 100.261.01 0.00 0.210.00 1.77 0.010 4 89.55 10.45 2384-1-ol2 rim 0.71 41.2110.350.11 48.280.48 100.431.01 0.00 0.210.00 1.76 0.010 4 89.26 10.74 2384-3-ol1 ctr. med. subh. 0.72 40.8210.9 0.09 47.980.26 100.051.00 0.00 0.220.00 1.76 0.010 4 88.69 11.31 rim 0.72 41.299.66 0.09 49.140.3 100.481.01 0.00 0.200.00 1.78 0.010 4 90.07 9.93 2384-3-ol2 ctr. sm. euh. 0.72 41.219.85 0.1 48.760.3 100.221.01 0.00 0.200.00 1.78 0.010 4 89.82 10.18 2384-3-ol3 ctr. lge. anh. 0.72 40.6811.150.09 47.930.49 100.341.00 0.00 0.230.00 1.76 0.010 4 88.45 11.55 2384-9-ol1 ctr. sm. euh. 0.71 39.9911.7 0.12 47.150.28 99.241.00 0.00 0.240.00 1.75 0.010 4 87.78 12.22 2384-9-ol2 ctr. sm. euh. 0.71 40.739.95 0.1 48.390.28 99.451.00 0.00 0.210 1.78 0.010 4 89.66 10.34 2384-9-ol3 ctr. sm. euh. 0.71 40.7 10.560.06 48.680.26 100.261.00 0.00 0.220 1.78 0.010 4 89.15 10.85

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187Table B-1. Continued. Sample # Description Mg* SiO2FeOTMnOMgO CaOTotal Si Ti Fe MnMgCa CrOFo Fa 2384-9-ol4 ctr. sm. euh. 0.71 40.579.910.0948.17 0.33 99.07 1.00 0.00 0.210.001.780.010 4 89.6510.35 2384-9-ol5 ctr. sm. euh. 0.71 40.7610.120.0548.61 0.27 99.81 1.00 0.00 0.210.001.780.010 4 89.5410.46 2384-11-ol1 ctr. lge. anh. 0.67 40.4711.40.1347.77 0.25 100.021.00 0.00 0.240.001.760.010 4 88.1911.81 2386-5-ol1 ctr. sm. euh. 0.64 39.7312.920.1546.03 0.36 99.19 1.00 0.00 0.270.001.720.010 4 86.3913.61 2386-7-ol1 ctr. sm. euh. 0.58 38.715.420.2144.02 0.34 98.69 0.99 0.00 0.330.001.680.010 4 83.5716.43 2387-1-ol1 ctr. sm. euh. 0.58 40.1714.150.1445.48 0.28 100.221.00 0.00 0.300.001.690.010 4 85.1414.86 2387-6-ol1 ctr. sm. euh. 0.59 39.4415.260.1944.83 0.32 100.040.99 0.00 0.320.001.680.010 4 83.9616.04 2387-6-ol2 ctr. sm. euh. 0.59 39.6115.290.2144.65 0.34 100.1 1.00 0.00 0.320.001.670.010 4 83.8816.12 2388-3a-ol1 ctr. sm. euh. 0.57 39.3115.30.2 44.72 0.27 99.8 0.99 0.00 0.320.001.680.010 4 83.8916.11 rim 0.57 39.5115.70.1743.94 0.32 99.64 1.00 0.00 0.330.001.660.010 4 83.3016.70 2388-3a-ol2 ctr. med. euh. 0.57 39.6115.860.1844.13 0.27 100.051.00 0.00 0.330.001.660.010 4 83.2216.78 2388-10-ol1 ctr. sm. euh. 0.65 39.9413.080.1446.49 0.3 99.95 0.99 0.00 0.270.001.730.010 4 86.3713.63 2389-1-ol1 ctr. sm. euh. 0.57 39.615.770.1743.96 0.3 99.8 1.00 0.00 0.330.001.650.010 4 83.2416.76 2389-1-ol2 ctr. sm. euh 0.57 39.5815.980.1543.73 0.29 99.73 1.00 0.00 0.340.001.650.010 4 82.9817.02 2390-9-ol1 ctr. wormy 0.44 38.3824.020.2938.29 0.28 101.260.99 0.00 0.520.011.480.010 4 73.9626.04 50% outward 0.44 38.3323.90.3338.43 0.26 101.250.99 0.00 0.520.011.480.010 4 74.1325.87 75% outward 0.44 38.5123.780.3138.57 0.28 101.450.99 0.00 0.510.011.480.010 4 74.3025.70 rim 0.44 38.323.620.3 38.58 0.32 101.120.99 0.00 0.510.011.490.010 4 74.4325.57 D1-5-ol1 ctr. sm. euh. 0.62 40.1213.90.1445.86 0.33 100.351.00 0.00 0.290.001.700.010 4 85.4614.54 D4-2-ol1 ctr. sm. euh. 0.57 39.316.180.2 43.53 0.28 99.49 1.00 0.00 0.340.001.650.010 4 82.7417.26 D20-7-ol1 ctr. sm. euh. 0.71 40.699.940.0548.68 0.26 99.62 1.00 0.00 0.200.001.790.010 4 89.7210.28 D20-7-ol2 ctr. lge. anh. 0.71 41.059.270.1 48.72 0.23 99.37 1.01 0.00 0.190.001.780.010 4 90.359.65 50% outward 0.71 40.99.360.0949.12 0.25 99.72 1.00 0.00 0.190.001.790.010 4 90.349.66 D20-8-ol1 ctr. sm subh. 0.71 40.4810.210.1 48.46 0.27 99.52 1.00 0.00 0.210.001.780.010 4 89.4310.57 D20-30-ol1 ctr. sm. euh. 0.71 41.0410.510.1349.5 0.26 101.440.99 0.00 0.210.001.790.010 4 89.3510.65 D20-30-ol2 ctr. lge. anh. 0.71 40.839.920.0548.45 0.28 99.53 1.00 0.00 0.200.001.780.010 4 89.7010.30 D20-30-ol3 ctr. lge. anh. 0.71 41 8.970.0849.12 0.25 99.42 1.01 0.00 0.180.001.800.010 4 90.719.29 D20-30-ol4 ctr. lge. anh. 0.71 40.969.980.0848.66 0.28 99.96 1.00 0.00 0.200.001.780.010 4 89.6810.32 30% outward 0.71 40.7310.010.0848.58 0.25 99.65 1.00 0.00 0.210.001.780.010 4 89.6410.36 60% outward 0.71 40.6910.010.0948.72 0.26 99.77 1.00 0.00 0.210.001.790.010 4 89.6610.34

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188Table B-1. Continued. Sample # Description Mg* SiO2 FeOT MnOMgOCaOTotal Si Ti Fe MnMgCa CrO Fo Fa rim 0.71 40.8610.110.08 48.890.29 100.231.00 0.000.210.001.780.010 4 89.6010.40 D20-30-ol5 ctr. lge. anh. 0.71 40.869.53 0.09 49.070.26 99.81 1.00 0.00 0.200.001.790.010 4 90.179.83 50% outward 0.71 41.049.52 0.03 48.9 0.25 99.74 1.01 0.00 0.200.001.790.010 4 90.159.85 rim 0.71 41.3 9.6 0.1 48.880.26 100.141.01 0.00 0.200.001.780.010 4 90.079.93 D20-30-ol6 ctr. lge. anh. 0.71 41.059.98 0.07 48.8 0.31 100.211.00 0.00 0.200.001.780.010 4 89.7110.29 D20-30-ol7 ctr. lge. anh. 0.71 40.969.98 0.08 48.660.28 99.96 1.00 0.00 0.200.001.780.010 4 89.6810.32 30% outward 0.71 40.7310.010.08 48.580.25 99.65 1.00 0.00 0.210.001.780.010 4 89.6410.36 60% outward 0.71 40.6910.010.09 48.720.26 99.77 1.00 0.00 0.210.001.790.010 4 89.6610.34 rim 0.71 40.8610.110.08 48.890.29 100.231.00 0.00 0.210.001.780.010 4 89.6010.40 D20-30-ol8 ctr. lge. anh. 0.71 40.869.53 0.09 49.070.26 99.81 1.00 0.00 0.200.001.790.010 4 90.179.83 50% outward 0.71 41.049.52 0.03 48.9 0.25 99.74 1.01 0.00 0.200.001.790.010 4 90.159.85 rim 0.71 41.3 9.6 0.1 48.880.26 100.141.01 0.00 0.200.001.780.010 4 90.079.93 D20-30-ol9 ctr. lge. anh. 0.71 41.059.98 0.07 48.8 0.31 100.211.00 0.00 0.200.001.780.010 4 89.7110.29 D21-1-ol1 ctr. lge. anh. 0.63 40.1514.190.14 45.630.24 100.351.00 0.00 0.300.001.690.010 4 85.1414.86 rim 0.63 40.1 14.030.1 46.080.24 100.551.00 0.00 0.290.001.710.010 4 85.4114.59 D21-1-ol2 ctr. lge. anh. 0.63 40.2314.090.16 45.790.26 100.531.00 0.00 0.290.001.700.010 4 85.2814.72 D21-1-ol3 ctr. lge. anh. 0.63 40.1714.430.13 45.370.21 100.311.00 0.00 0.300.001.690.010 4 84.8615.14 D22-3-ol1 ctr. sm. euh. 0.69 40.6310.180.06 48.750.24 99.86 1.00 0.00 0.210.001.790.010 4 89.5110.49 D22-4-ol1 ctr. lge. anh. 0.68 41.298.88 0.08 49.460.24 99.95 1.01 0.00 0.180.001.800.010 4 90.859.15 D26-6-ol1 ctr. sm. anh. 0.56 39.1715.750.18 43.430.34 98.87 1.00 0.00 0.340.001.650.010 4 83.0916.91 D26-6-ol2 ctr. sm. euh. 0.56 39.3516.030.18 43.520.34 99.42 1.00 0.00 0.340.001.650.010 4 82.8717.13 D27-5-ol1 ctr sm. euh. att.spin. 0.69 40.3710.890.12 47.670.27 99.32 1.00 0.00 0.230.001.760.010 4 88.6411.36 D27-5-ol2 ctr. sm. euh. att. spin. 0.69 40.5210.690.09 48.180.25 99.73 1.00 0.00 0.220.001.770.010 4 88.9311.07 2384-4a ol 0.61 40.22212.6380.34546.1160.29699.6171.00 0.00 0.2640.0071.710.010 4 86.6713.33 Marj olv 0.61 39.65910.910.28743.250.04994.1551.03 0.00 0.2380.0061.680.000 4 87.6012.40 2384-4a ol core 0.61 39.9611.9070.29746.190.27798.6311.00 0.00 0.250.0061.730.010 4 87.3612.64 ol rim 0.61 39.73518.3440.28 42.2860.281100.9261.00 0.00 0.3880.0061.590.010 4 80.4219.58 2384-4a ol in clot 0.61 40.34113.5430.16345.9620.266100.2751.00 0.00 0.2820.0031.700.010 4 85.8114.19

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189Table B-1. Continued. Sample # Description Mg* SiO2FeO MnOMgO CaO TotalSi Ti Fe Mn MgCaCrOFo Fa 2384-4a big ol 0.61 38.17416.5970.18342.8660.26 98.080.99 0.000.3590.0041.650.010 4 82.1517.85 2384-4a big ol 0.61 38.28716.2270.36142.6350.28497.7940.99 0.00 0.3520.0081.650.010 4 82.4017.60 big ol 0.61 38.73315.7460.34943.2460.28798.3610.99 0.00 0.3380.0081.660.010 4 83.0316.97 big ol nearer edge 0.61 38.39317.0340.25142.6470.29698.6210.99 0.00 0.3670.0051.640.010 4 81.6918.31 2384-4a ol next to plag 0.61 37.71516.5850.16242.5860.27497.3220.98 0.00 0.3620.0041.660.010 4 82.06 17.94 Notes: Mg = 100 Mg/(Mg + Fe2+) of the host glass. Oxides expressed in wt. %. Fo = fosterite content. Fa = fayalite content.

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190Table B-2. Microprobe analys is of plagioclase phenocrysts in the Siqueiros samples. Sample Description Mg* SiO2 Al2O3 FeOMgOCaONa2OK2OTotalSi Al Fe Mg CaNaK OAn Ab Or 2375-7-pl1 ctr. sm lath 0.62 51.6629.480.670.15 13.583.540.0199.092.37 1.590.030.010.670.310.001867.9132.030.06 2375-7-pl2 ctr. sm. lath 51.0329.710.680.18 13.913.280.0198.8 2.34 1.600.030.010.680.290.001870.0529.890.06 2375-9-pl1 ctr. sm. lath 0.6 51.229.630.890.66 13.853.5 0.0299.752.36 1.610.030.050.680.310.001868.5431.340.12 2375-9-pl2 ctr. lge. subh. phen. 51.4928.960.920.28 13.253.7 0.0198.612.38 1.580.040.020.660.330.001866.3933.550.06 rim 51.7128.490.980.35 13.013.740.0298.3 2.38 1.540.040.020.640.330.001865.7034.180.12 2375-9-pl3 ctr. lge. euh. phen. 51.7529.7 0.671.21 13.563.52 100.412.36 1.600.030.080.660.310.000868.0431.960.00 2376-3-pl1 ctr. sm plag lath 0.62 51.4230.290.580.2 14.043.320.0199.862.34 1.620.020.010.680.290.001869.9929.950.06 2376-3-pl2 ctr. lge. anh. phen. 49.7531.650.450.18 15.072.79 99.892.28 1.710.020.010.740.250.000874.9025.100.00 2376-8-pl1 ctr. sm lath 0.61 49.7731.880.470.11 15.172.690.01100.12.27 1.710.020.010.740.240.001875.6624.280.06 2377-3-pl1 ctr. lge anh. phen. 0.61 48.6632.230.270.15 15.772.43 99.512.23 1.740.010.010.780.220.000878.2021.800.00 50% outward 49.132.010.650.12 15.992.18 100.052.25 1.730.020.010.790.190.000880.2119.790.00 75% outward 49.2931.820.330.17 15.42.52 99.532.25 1.710.010.010.750.220.000877.1522.850.00 rim 47.333.670.370.06 17.31.52 100.222.17 1.820.010.000.850.140.000886.2813.720.00 2377-3-pl2 ctr. lge. anh. phen. 46.7233.430.310.04 17.221.53 99.252.17 1.830.010.000.860.140.000886.1513.850.00 50% outward 47.7633.4 0.280.11 16.691.79 100.032.19 1.800.010.010.820.160.000883.7516.250.00 65% outward 47.5933.510.270.1 16.761.73 99.962.18 1.810.010.010.820.150.000884.2615.740.00 75% outward 46.5433.870.220.04 17.181.54 99.392.15 1.850.010.000.850.140.000886.0413.960.00 90% outward 47.9632.870.230.08 16.42.03 99.572.20 1.780.010.010.810.180.000881.7018.300.00 rim 47.0133.780.350.09 16.851.63 99.712.17 1.840.010.010.830.150.000885.1014.900.00 2377-3-pl3 ctr. lge. anh. phen. 50.0431.680.320.13 15.542.630.01100.352.27 1.690.010.010.760.230.001876.5123.430.06 25% outward 48.1733.690.310.06 16.751.93 100.912.19 1.800.010.000.810.170.000882.7517.250.00 50% outward 47.4434.040.310.04 17.251.63 100.712.17 1.830.010.000.840.140.000885.4014.600.00 75% outward 49.6732.060.390.16 15.732.6 0.01100.622.25 1.710.010.010.760.230.001876.9323.010.06 rim 47.3833.760.410.09 17.351.67 100.662.19 1.840.020.010.860.150.000885.1714.830.00 2377-3-pl4 ctr. sm lath 52.5230.170.5 0.13 13.683.640.02100.662.37 1.610.020.010.660.320.001867.4232.460.12 rim 51.8230.1 0.570.09 13.773.6 0.0299.972.36 1.610.020.010.670.320.001867.8032.080.12 2377-3-pl5 ctr. sm. lath 51.9530.690.520.14 14.013.5 0.01100.822.34 1.630.020.010.680.310.001868.8331.120.06 rim 51.2130.7 0.560.14 14.453.110.01100.182.33 1.650.020.010.700.270.001871.9328.010.06 2377-3-pl6 ctr. tiny lath 51.3630.6 0.830.13 14.083.380.01100.392.34 1.640.030.010.690.300.001869.6730.270.06 2377-4-pl1 ctr. tiny lath 0.59 52.0929.930.860.19 13.533.570.02100.192.36 1.600.030.010.660.310.001867.6032.280.12 2377-11pl1 ctr. tiny lath 0.56 51.1430.460.770.19 13.983.4 0.0299.962.34 1.650.030.010.690.300.001869.3630.520.12

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191Table B-2. Continued. Sample Description Mg* SiO2 Al2O3FeOMgOCaONa2OK2OTotalSi Al Fe Mg CaNaK OAn Ab Or 2377-11-pl2 ctr med. euh. 52.2229.570.610.1 13.193.740.0199.442.38 1.590.020.010.640.330.0018 66.0533.890.06 rim 51.2130.360.790.0614.073.32 99.812.34 1.630.030.000.690.290.0008 70.0829.920.00 2377-11-pl3 ctr. lge anh. phen. 50.0131.680.350.1915.122.56 99.912.27 1.700.010.010.740.230.0008 76.5523.450.00 10% outward 47.2733.30.220.0316.951.75 99.522.18 1.810.010.000.840.160.0008 84.2615.740.00 20% outward 47.4633.140.250.0716.891.71 99.522.18 1.800.010.000.830.150.0008 84.5215.480.00 30% outward 47.2233.540.280.0717.251.53 99.892.18 1.820.010.000.850.140.0008 86.1713.830.00 50% outward 47.8333.40.270.0716.541.81 99.922.20 1.810.010.000.810.160.0008 83.4716.530.00 75% outward 48.8732.360.270.1115.712.34 99.662.24 1.750.010.010.770.210.0008 78.7721.230.00 85% outward 47.7432.760.330.0916.231.96 99.112.22 1.800.010.010.810.180.0008 82.0717.930.00 rim 51.7930.590.560.0913.53.550.02100.12.34 1.630.020.010.650.310.0018 67.6832.200.12 2378-6-pl1 ctr. sm ueh. 51.6530.230.780.1613.943.460.01100.232.35 1.620.030.010.680.310.0018 68.9630.980.06 2378-6-pl2 ctr. sm. euh. 50.5731.060.490.1214.512.96 99.712.31 1.680.020.010.710.260.0008 73.0426.960.00 2380-4-pl1 ctr. sm. lath 0.59 51.5130.170.530.1413.753.440.0299.562.34 1.610.020.010.670.300.0018 68.7531.130.12 2380-11-pl1 ctr. lge. anh. phen. 0.58 46.6133.980.230.0917.071.52 99.5 2.15 1.850.010.010.840.140.0008 86.1213.880.00 50% outward 46.7833.980.270.0917.271.47 99.862.17 1.860.010.010.860.130.0008 86.6513.350.00 95% outward 50.5731.130.540.1 14.43.120.0199.872.31 1.680.020.010.700.280.0018 71.7928.150.06 rim 50.1331.210.610.0914.512.95 99.5 2.30 1.690.020.010.710.260.0008 73.1026.900.00 2380-11-pl2 ctr. sm. lath 51.3 30.430.660.1213.913.280.0299.722.34 1.640.030.010.680.290.0018 70.0129.870.12 2380-11-pl3 ctr. sm. lath 51.4530.280.7 0.0713.733.470.0299.722.35 1.630.030.000.670.310.0018 68.5431.340.12 rim 51.3830.430.760.0913.833.4 0.0199.9 2.33 1.620.030.010.670.300.0018 69.1730.770.06 2380-11-pl4 ctr. sm. euh. 48.2332.460.580.0416.392.04 99.742.22 1.760.020.000.810.180.0008 81.6218.380.00 2380-11-pl5 ctr. med. anh. phen. 48.4416.440.380.1 16.441.95 83.752.68 1.070.020.010.970.210.0008 82.3317.670.00 2381-14apl1 ctr. sm. lath 0.66 51.7129.470.590.2213.613.480.0199.092.37 1.590.020.020.670.310.0018 68.3331.610.06 2382-7-pl1 ctr. sm lath 0.58 52.2729.550.7 0.1513.343.740.0299.772.41 1.600.030.010.660.330.0018 66.2633.620.12 2382-7-pl2 ctr. glomero. w cpx+ol 53.4 29.450.590.1412.912.57 99.062.40 1.560.020.010.620.220.0008 73.5226.480.00 2382-7-pl2 ctr.glomero.w. cpx+ol 51.0230.340.590.0914.113.290.0199.452.34 1.640.020.010.690.290.0018 70.2829.660.06 2383-2-pl1 ctr. sm. lath 0.59 51.6930.60.520.1 13.773.450.01100.142.34 1.640.020.010.670.300.0018 68.7631.180.06 2383-2-pl2 ctr. sm. lath 52.2930.420.6 0.1813.653.570.02100.732.34 1.600.020.010.650.310.0018 67.8032.090.12 2383-6-pl1 ctr. med subh. 0.67 46.9434.020.250.1117.231.58 100.132.16 1.840.010.010.850.140.0008 85.7714.230.00

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192Table B-2. Continued. Sample Description Mg* SiO2 Al2O3FeOMgOCaONa2OK2OTotal Si Al Fe Mg Ca NaK OAn Ab Or 30% outward 47.2533.8 0.260.0417.21.54 100.09 2.17 1.830.010.000.850.140.0008 86.0613.940.00 60% outward 47.4133.930.240.0917.051.59 100.31 2.17 1.830.010.010.830.140.0008 85.5614.440.00 rim 47.4633.940.340.1117.021.63 100.5 2.18 1.840.010.010.840.140.0008 85.2314.770.00 2383-6-pl2 ctr. sm lath 50.2431.760.4 0.2115.082.74 100.43 2.29 1.710.020.010.740.240.0008 75.2624.740.00 2386-5-pl1 ctr. tiny lath 0.64 50.4 30.530.720.2113.983.160.0199.01 2.32 1.660.030.010.690.280.0018 70.9329.010.06 2386-7-pl1 ctr. tiny lath 0.58 52.2930.430.730.1713.433.540.01100.6 2.34 1.610.030.010.640.310.0018 67.6632.280.06 2387-1-pl1 ctr. lge anh. phen. 0.64 46.3334.990.2 0.0617.431.35 100.36 2.12 1.890.010.000.860.120.0008 87.7112.290.00 50% outward 47.0134.260.210.0616.971.630.06100.2 2.15 1.850.010.000.830.140.0048 84.8914.750.36 75% outward 46.5634.050.230.0517.441.4 99.73 2.14 1.850.010.000.860.130.0008 87.3212.680.00 90% outward 46.5134.510.3 0.0717.351.45 100.19 2.14 1.870.010.000.850.130.0008 86.8613.140.00 95% outward 46.6934.550.240.0617.221.42 100.18 2.15 1.870.010.000.850.130.0008 87.0212.980.00 2387-1-pl2 ctr. lge. anh. phen. 48.8531.970.290.1 15.732.38 99.32 2.25 1.730.010.010.780.210.0008 78.5121.490.00 50% outward 49.0832.610.320.0915.932.3 100.33 2.25 1.760.010.010.780.200.0008 79.2820.720.00 2387-1-pl2 ctr. sm. lath 51.0630.6 0.590.1214.053.240.0199.67 2.31 1.640.020.010.680.280.0018 70.5129.430.06 2387-1-pl3 ctr. lge anh. phen. 46.9833.640.230.0517.271.53 99.7 2.18 1.840.010.000.860.140.0008 86.1813.820.00 rim 50.2 31.3 0.350.1214.822.7 99.49 2.28 1.680.010.010.720.240.0008 75.2124.790.00 2387-1-pl4 ctr. lge anh. phen. 46.3533.890.230.0717.391.4 99.33 2.15 1.850.010.000.860.130.0008 87.2812.720.00 50% outward 46.2733.910.2 0.0417.421.32 99.16 2.14 1.850.010.000.870.120.0008 87.9412.060.00 75% outward 47.2633.740.250.0517.141.51 99.95 2.17 1.830.010.000.840.130.0008 86.2513.750.00 rim 47.7533.130.280.0916.411.92 99.58 2.21 1.810.010.010.810.170.0008 82.5317.470.00 2387-1-pl5 ctr. med. euh. 51.1230.690.490.1113.963.42 99.79 2.33 1.650.020.010.680.300.0008 69.2830.720.00 2387-6-pl1 ctr. sm. lath 0.59 51.6430.160.730.1613.53.570.0199.77 2.35 1.620.030.010.660.320.0018 67.5932.350.06 rim 51.9929.890.810.1813.483.630.0199.99 2.37 1.600.030.010.660.320.0018 67.2032.740.06 2387-6-pl2 ctr. lge. wormy anh. 49.3130.910.370.1 14.842.72 98.25 2.28 1.690.010.010.740.240.0008 75.0924.910.00 10% outward 48.9632.070.4 0.0715.562.410.0199.48 2.25 1.740.020.000.770.210.0018 78.0621.880.06 2387-6-pl3 ctr. lge. wormy anh. 47.9132.7 0.370.0615.992.11 99.14 2.21 1.780.010.000.790.190.0008 80.7219.280.00 20% outward 48.8932.230.330.1 15.552.39 99.49 2.24 1.740.010.010.760.210.0008 78.2421.760.00 40% outward 48.5232.2 0.340.0715.732.39 99.25 2.23 1.750.010.000.780.210.0008 78.4321.570.00 60% outward 48.1232.9 0.3 0.0916.272.12 99.8 2.21 1.780.010.010.800.190.0008 80.9219.080.00 80% outward 48.4432.340.390.1215.952.31 99.55 2.24 1.760.020.010.790.210.0008 79.2320.770.00 2388-3a-pl1 ctr. sm lath 0.57 51.7 30.920.550.1713.823.370.01100.54 2.34 1.650.020.010.670.300.0018 69.3430.600.06 rim 51.8130.6 0.760.1713.813.33 100.48 2.33 1.620.030.010.670.290.0008 69.6230.380.00 2388-3a-pl2 ctr. lge. phen. 48.0933.6 0.3 0.0816.641.9 100.61 2.19 1.810.010.010.810.170.0008 82.8817.120.00

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193Table B-2. Continued. Sample Description Mg* SiO2Al2O3FeOMgOCaONa2OK2OTotal Si Al Fe Mg CaNaK OAn Ab Or 50% outward 48.2233.460.260.0616.471.93 100.4 2.20 1.800.010.000.810.170.0008 82.5017.500.00 rim 48.433.350.320.0716.182.06 100.38 2.20 1.780.010.000.790.180.0008 81.2718.730.00 2388-3a-pl3 ctr. med. euh. 46.7335.260.370.1117.641.29 101.4 2.12 1.890.010.010.860.110.0008 88.3111.690.00 50% outward 46.9434.350.380.1 17.451.39 100.61 2.17 1.870.010.010.860.120.0008 87.4012.600.00 rim 51.9830.5 0.580.0913.643.410.01100.21 2.34 1.620.020.010.660.300.0018 68.8131.130.06 2388-3a-pl4 ctr. lge. euh. 46.7634.150.280.0417.141.52 99.89 2.15 1.850.010.000.840.140.0008 86.1713.830.00 50% outward 46.9733.880.270.1 17.231.68 100.13 2.16 1.840.010.010.850.150.0008 85.0015.000.00 75% outward 47.3333.810.280.0817 1.63 100.13 2.17 1.830.010.010.840.140.0008 85.2114.790.00 rim 47.1234.4 0.340.0716.831.67 100.43 2.16 1.860.010.000.830.150.0008 84.7815.220.00 2388-3a-pl5 ctr. med. euh. 49.8932.340.6 0.1214.962.670.01100.59 2.27 1.730.020.010.730.240.0018 75.5424.400.06 50% outward 49.2932.590.4 0.1 15.072.590.01100.05 2.25 1.750.020.010.740.230.0018 76.2323.710.06 75% outward 49.8432.580.5 0.0915.12.59 100.7 2.25 1.730.020.010.730.230.0008 76.3123.690.00 2388-3a-pl6 ctr. med. euh. 47.7833.8 0.340.0816.571.87 100.44 2.20 1.830.010.010.820.170.0008 83.0416.960.00 rim 51.7530.350.650.2213.693.350.02100.03 2.36 1.630.020.010.670.300.0018 69.2330.650.12 2388-3a-pl7 ctr. lge. anh. phen. 53.0429.930.550.0512.83.940.02100.33 2.40 1.590.020.000.620.350.0018 64.1535.730.12 50% outward 52.4130.180.520.0412.74 0.0299.87 2.38 1.610.020.000.620.350.0018 63.6236.260.12 75% outward 52.0930.520.540.0512.963.820.0199.99 2.35 1.620.020.000.630.330.0018 65.1834.760.06 rim 49.2432.710.430.1115.012.68 100.18 2.24 1.760.020.010.730.240.0008 75.5824.420.00 2388-10-pl1 ctr. lge lath 0.65 49.8331.690.390.1514.882.81 99.75 2.28 1.710.010.010.730.250.0008 74.5325.470.00 2388-10-pl2 ctr. lge anh. phen 50.5931.150.510.2314.653 0.01100.14 2.30 1.670.020.020.710.260.0018 72.9227.020.06 2388-10-pl3 ctr. lge. euh. 50.1331.740.520.2114.792.88 100.27 2.28 1.700.020.010.720.250.0008 73.9426.060.00 50% outward 50.3831.440.550.1514.722.82 100.06 2.29 1.690.020.010.720.250.0008 74.2625.740.00 75% outward 50.4931.490.540.2514.92.8 100.47 2.31 1.690.020.020.730.250.0008 74.6225.380.00 2389-1-pl1 ctr. med. lath 0.57 51.4330.590.470.1313.783.31 99.71 2.34 1.640.020.010.670.290.0008 69.7030.300.00 2390-9-pl1 ctr. lge anh. 0.44 50.9230.830.530.0614.33.240.0199.89 2.34 1.670.020.000.700.290.0018 70.8829.060.06 rim 53.6728.830.740.0311.954.430.0299.67 2.43 1.540.030.000.580.390.0018 59.7840.100.12 50% outward 51.8230.550.490.0213.553.580.01100.02 2.36 1.640.020.000.660.320.0018 67.6132.330.06 2390-9-pl2 ctr. lge. subh. 54.0428.550.730.0511.864.610.0399.87 2.45 1.530.030.000.580.410.0028 58.6041.220.18 2390-9-pl3 ctr. sm. lath 53.328.010.750.0211.574.610.0498.3 2.46 1.520.030.000.570.410.0028 57.9741.800.24 rim 52.8728.35 0.79 0.02 11.774.36 0.0298.18 2.42 1.530.030.00 0.580.390.0018 59.8040.080.12 D1-5-pl1 ctr. tiny lath 0.62 52.2 30.33 0.65 0.27 13.693.56 0.02100.72 2.36 1.620.020.02 0.660.310.0018 67.9231.960.12 D4-2-pl1 ctr. sm. lath 0.57 51.9730.59 0.56 0.1 13.713.5 0.01100.44 2.35 1.630.020.01 0.660.310.0018 68.3631.580.06 D4-2-pl2 ctr. sm. lath 52.3530.23 0.66 0.11 13.693.47 0.02100.53 2.37 1.610.020.01 0.660.300.0018 68.4731.410.12

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194Table B-2. Continued. Sample Description Mg* SiO2Al2O3FeOMgOCaONa2OK2OTotal Si Al Fe Mg Ca NaK OAn Ab Or D4-2-pl3 ctr. lge. anh. phen 51.7330.490.5 0.11 13.713.430.0299.99 2.34 1.630.020.010.660.300.001868.7531.130.12 D13-1-pl1 ctr. lge anh. phen 0.58 49.0632.4 0.360.08 15.652.31 99.86 2.24 1.740.010.010.760.200.000878.9221.080.00 50% outward 46.8434.050.330.07 17.071.52 99.88 2.16 1.850.010.000.840.140.000886.1213.880.00 75% outward 48.6432.820.540.09 15.832.2 100.12 2.22 1.770.020.010.780.200.000879.9020.100.00 95% outward 48.7432.590.440.09 15.82.24 99.9 2.23 1.760.020.010.780.200.000879.5820.420.00 D15-1-pl1 ctr. lge anh. phen. 0.58 48.2332.660.350.09 15.912.17 99.41 2.22 1.770.010.010.790.190.000880.2019.800.00 25% outward 48.4532.740.330.06 15.832.16 99.57 2.23 1.780.010.000.780.190.000880.2019.800.00 50% outward 49.9 31.390.410.13 14.433.020.0199.29 2.28 1.690.020.010.710.270.001872.4927.450.06 75% outward 48.7332.710.4 0.07 15.862.22 99.99 2.23 1.760.020.000.780.200.000879.7920.210.00 95% outward 48.2632.840.530.06 15.82.21 99.7 2.22 1.780.020.000.780.200.000879.8020.200.00 rim 48.9232.110.620.07 15.32.42 99.44 2.24 1.740.020.000.750.220.000877.7522.250.00 D17-10-pl1 ctr med. euh. 0.61 47.4 33.220.470.09 16.371.81 99.36 2.20 1.820.020.010.810.160.000883.3316.670.00 50% outward 49.2331.3 0.510.14 14.742.69 98.61 2.27 1.700.020.010.730.240.000875.1724.830.00 rim 47.9332.5 0.510.08 16.012.03 99.06 2.24 1.790.020.010.800.180.000881.3418.660.00 D26-6-pl1 ctr. med euh. 0.56 52.8228.980.8 0.12 12.414.060.0399.22 2.42 1.560.030.010.610.360.002862.7037.120.18 D26-6-pl2 ctr. med euh. 51.9229.210.620.12 12.853.650.0298.39 2.39 1.580.020.010.630.330.001865.9733.910.12 D26-6-pl3 Ctr sm. lath 52.2830.260.590.12 13.373.670.02100.31 2.65 1.810.020.010.730.360.001866.7333.150.12 2384-4a plag in clot core 0.61 46.6133.510.330.20 17.441.570.0199.65 2.15 1.820.010.010.860.140.000886.0013.990.02 plag in clot rim 0.61 48.1729.440.560.20 14.653.240.0296.28 2.28 1.640.020.010.740.300.001871.4128.570.05 smaller plag c in ol 0.61 51.3630.280.560.21 14.193.490.02100.10 2.32 1.610.020.010.690.310.001869.2430.740.07 smaller plag rim in ol 0.61 51.0030.420.580.20 14.273.300.0299.79 2.31 1.630.020.010.690.290.001870.4629.520.07 med plag core in ol 0.61 50.1430.340.490.20 14.743.190.0299.12 2.29 1.640.020.010.720.280.001871.8628.120.07 med plag rim in ol 0.61 49.1729.810.580.17 14.643.170.0297.56 2.29 1.640.020.010.730.290.001871.8628.130.07 p lag pheno in matrix core 0.61 50.0230.390.440.18 14.543.140.0398.74 2.29 1.640.020.010.710.280.001871.9028.080.07 p lag pheno in matrix rim 0.61 46.3430.650.520.14 15.952.530.0296.14 2.21 1.720.020.010.810.230.001877.7122.280.05 plag mega core 0.61 46.8833.570.310.15 17.471.650.01100.03 2.15 1.820.010.010.860.150.000885.4314.570.02 plag mega half rim 0.61 45.8332.300.300.14 17.431.670.0197.68 2.16 1.790.010.010.880.150.000885.2614.730.03 plag mega rim 0.61 48.7827.980.700.21 13.323.960.0494.99 2.32 1.570.030.010.680.370.001865.0134.950.10 Notes: Mg = 100 Mg/(Mg + Fe2+) of the host glass. Oxides expressed in wt. %. An = anorthite content. Ab = albite content Or = Orthoclase content.

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195Table B-3. Microprobe analys is of spinel phenocrysts in the Siqueiros samples. Sample # Description Mg* TiO2 Al2O3 Cr2O3Fe2O3FeOMnOMgONiOCaOTotalTiAl Cr Fe3+Fe2+MnMgNi CaTETOCTOFE*CR* D20-8-sp1 ctr. lg. wormy 0.71 0.14 44.09 22.50 2.98 8.78 0.02 19.240.190.0297.96 0.0211.50 3.940.53 1.62 0.006.340.030.0015.988.01 3220.3 825.50 20% outward 0.71 0.16 44.67 23.54 2.97 9.57 0.02 19.340.160.01100.440.0311.41 4.030.52 1.73 0.006.250.030.0015.988.01 3221.7326 .12 40% outward 0.71 0.16 44.87 23.32 2.92 9.56 0.02 19.350.170.01100.380.0311.45 3.990.51 1.73 0.006.250.030.0015.988.01 3221.7025 .85 60% outward 0.71 0.19 44.95 23.07 2.94 9.52 0.02 19.400.120.01100.210.0311.48 3.950.51 1.72 0.006.270.020.0015.988.02 3221.5825 .61 rim 0.71 0.16 43.70 22.26 2.35 9.20 0.04 18.660.140.0296.53 0.0311.57 3.950.42 1.73 0.016.250.030.0015.988.02 3221.6725.47 D20-8-sp2 ctr. lg wormy 0.71 0.24 39.78 28.74 3.34 9.78 0.08 18.860.050.02100.900.0410.33 5.010.59 1.80 0.016.190.010.0015.978.03 3222.54 32.64 60% outward 0.71 0.19 38.97 28.53 4.04 9.71 0.02 18.590.180.05100.290.0310.21 5.020.72 1.81 0.006.160.030.0115.988.02 3222.6732 .93 D20-15-sp1 euh. within glass 0.71 0.49 37.35 28.75 4.38 10.130.03 18.140.090.1499.50 0.089.93 5.130.79 1.91 0.016.100.020.0315.938.06 3223.8634.05 D20-30-sp1 euh. within oliv. 0.71 0.22 40.56 27.02 3.54 9.75 0.15 18.600.190.08100.100.0410.58 4.730.63 1.80 0.036.140.030.0215.978.02 322 2.7330.88 D20-30-sp2 euh. within glass 0.71 0.31 38.83 28.52 4.48 10.230.10 18.270.150.29101.180.0510.13 4.990.79 1.89 0.026.030.030.0715.968.03 3223.9133.01 D20-30-sp3 euh. within glass 0.71 0.70 35.34 29.25 6.63 9.64 0.15 18.460.100.14100.410.129.38 5.211.20 1.82 0.036.200.020.0315.908.09 3222.6635.70 D20-30-sp1 ctr.lg. euh. within glass 0.71 0.14 43.50 24.87 2.81 9.61 0.04 19.160.100.02100.250.0211.18 4.290.49 1.75 0.016.230.020.0015.988.01 3221.962 7.72 50% outward 0.71 0.16 44.10 24.33 2.99 9.50 0.02 19.360.160.02100.640.0311.27 4.170.52 1.72 0.006.260.030.0015.988.01 3221.5927 .01 rim 0.71 0.14 45.32 22.33 3.15 9.37 0.02 19.400.170.0299.92 0.0211.59 3.830.55 1.70 0.006.270.030.0015.998.01 3221.3224.84 D20-30-sp2 ctr.lg. euh. within glass 0.71 0.22 36.22 32.35 2.92 10.210.02 18.070.070.01100.090.049.63 5.770.53 1.93 0.006.080.010.0015.978.03 3224.0837 .47 near center 0.71 0.21 37.20 31.05 2.88 10.020.04 18.200.050.0199.67 0.049.88 5.530.52 1.89 0.016.110.010.0015.978.02 3223.6135. 89 50% outward 0.71 0.19 39.07 29.23 3.24 9.62 0.06 18.720.090.01100.240.0310.23 5.140.58 1.79 0.016.200.020.0015.988.02 3222.3833 .42 75% outward 0.71 0.12 43.01 25.62 3.07 9.43 0.03 19.330.090.01100.720.0211.03 4.410.54 1.72 0.016.270.020.0015.998.01 3221.5028 .55 rim 0.71 0.18 44.36 23.42 3.28 9.44 0.03 19.330.190.02100.250.0311.36 4.020.57 1.71 0.016.260.030.0015.988.02 3221.5026.15 D20-30-sp3 ctr. subh. within oliv. 0.71 0.18 42.96 25.53 2.35 10.040.02 18.810.110.02100.020.0311.11 4.430.41 1.84 0.006.150.020.0015.988.02 3223.0528.50 50% outward 0.71 0.15 43.17 25.44 2.77 9.69 0.02 19.120.150.02100.530.0211.09 4.380.48 1.77 0.006.210.030.0015.988.01 3222.1428 .33 rim 0.71 0.18 44.56 22.89 2.83 9.72 0.02 19.030.150.0199.38 0.0311.49 3.960.50 1.78 0.006.210.030.0015.988.02 3222.2725.63 D20-30-sp4 euh. within oliv. 0.71 0.15 45.40 22.50 2.68 9.77 0.02 19.170.120.0299.84 0.0211.63 3.860.47 1.78 0.006.210.020.0015.988.01 322 2.2524.95 D20-30-sp5 ctr. round in oliv. 0.71 0.20 40.46 27.79 3.52 9.09 0.03 19.250.140.02100.490.0310.49 4.830.62 1.67 0.016.310.020.0015.988.02 3220.9431.54 50% outward 0.71 0.20 39.99 28.30 3.44 9.15 0.02 19.180.100.01100.380.0310.40 4.940.61 1.69 0.006.310.020.0015.988.02 3221.1132 .19 rim 0.71 0.24 39.95 28.17 3.13 9.49 0.02 18.880.140.01100.030.0410.44 4.940.56 1.76 0.006.240.020.0015.978.03 3222.0032.11 D21-1-sp1 ctr. lg. anhedra 0.63 0.34 21.95 44.31 5.24 15.180.08 13.520.010.01100.650.066.31 8.551.03 3.10 0.024.920.000.0015.958.04 3238. 6557.52

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196Table B-3. Continued. Sample # Description Mg* TiO2 Al2O3 Cr2O3Fe2O3FeOMnOMgONiOCaOTotalTiAl CrFe3+Fe2+MnMgNiCaTETOCTOFE*CR* D21-1-sp1 ctr. lg. anhedra 0.63 0.34 21.95 44.31 5.24 15.180.08 13.520.010.01100.650.066.31 8.551.03 3.10 0.024.920.000.0015.95 8.04 3238.6557.52 20% outward 0.63 0.41 26.46 39.67 5.38 15.030.10 14.250.010.01101.320.077.40 7.441.02 2.98 0.025.040.000.0015.948.05 3237.1850 .14 40% outward 0.63 0.50 37.72 27.25 5.29 14.390.11 15.720.120.08101.180.0810.03 4.860.96 2.72 0.025.290.020.0215.938.06 3233.933 2.64 60% outward 0.63 0.47 38.49 26.11 5.49 13.980.11 16.040.060.03100.780.0810.22 4.650.99 2.63 0.025.380.010.0115.948.06 3232.853 1.27 80% outward 0.63 0.47 37.47 26.59 5.79 13.900.06 15.920.020.05100.270.0810.03 4.771.05 2.64 0.015.390.000.0115.948.06 3232.883 2.25 rim 0.63 0.58 35.95 28.45 4.60 14.940.09 14.990.010.1099.710.109.77 5.190.85 2.88 0.025.150.000.0215.918.08 3235.8734.68 D21-1-sp2 ctr lg. euh. att. oliv. 0.63 0.94 30.78 32.06 6.59 14.770.08 14.810.060.15100.240.178.51 5.941.24 2.90 0.025.180.010.0415.858.14 3235.8841.13 50% outward 0.63 0.89 31.34 30.84 6.51 14.500.07 14.840.020.1299.120.168.72 5.751.23 2.86 0.015.220.000.0315.868.13 3235.4039. 76 75% outward 0.63 0.85 32.97 29.98 6.67 14.650.06 15.170.050.11100.510.159.00 5.491.24 2.84 0.015.240.010.0315.878.12 3235.1437 .89 rim 0.63 0.74 33.71 29.50 6.19 14.480.07 15.210.010.11100.020.139.21 5.401.15 2.81 0.015.250.000.0315.898.10 3234.8236.99 D22-2-sp1 euh. att. oliv. 0.69 0.46 32.70 33.29 4.77 11.880.10 16.620.110.0399.960.088.90 6.080.88 2.29 0.025.720.020.0115.938. 06 3228.6340.58 D22-2-sp2 euh. att. oliv. 0.69 0.60 31.46 33.48 5.30 11.560.11 16.640.020.0899.250.118.64 6.170.99 2.25 0.025.780.000.0215.918. 08 3228.0541.65 D22-3-sp1 euh. att. oliv. 0.69 0.53 31.80 32.98 5.65 11.640.01 16.610.010.1899.410.098.71 6.061.05 2.26 0.005.760.000.0415.928. 07 3228.2241.03 D22-3-sp2 lg. round within glass 0.69 0.18 27.40 41.40 3.61 10.020.02 17.250.010.0299.910.037.58 7.680.68 1.97 0.006.040.000.0115.988.02 3224.5950.34 25% outward 0.69 0.24 26.82 42.38 3.46 10.160.02 17.200.070.01100.370.047.41 7.860.65 1.99 0.006.010.010.0015.978.03 3224.9051 .46 40% outward 0.69 0.22 26.99 42.02 3.21 10.260.03 17.030.060.0199.830.047.50 7.830.61 2.02 0.015.980.010.0015.978.03 3225.2751. 08 50% outward 0.69 0.22 28.64 40.29 3.10 10.270.03 17.180.090.0199.830.047.90 7.450.58 2.01 0.015.990.020.0015.978.03 3225.1148. 55 75% outward 0.69 0.20 32.73 36.19 3.24 10.130.04 17.800.080.01100.430.038.81 6.540.59 1.94 0.016.060.010.0015.978.02 3224.2142 .58 90% outward 0.69 0.22 36.10 32.55 3.26 10.440.05 18.000.090.02100.740.049.56 5.780.59 1.96 0.016.030.020.0015.978.02 3224.5637 .69 rim 0.69 0.24 36.94 30.81 3.31 11.050.02 17.570.100.02100.060.049.83 5.500.60 2.09 0.005.910.020.0015.978.03 3226.0935.88 D22-3-sp3 ctr. lg anh. in glass 0.69 0.19 38.03 31.30 2.91 10.340.01 18.390.110.01101.290.039.94 5.490.52 1.92 0.006.080.020.0015.988.02 3223.9835.57 30% outward 0.69 0.23 35.46 33.91 3.14 9.74 0.02 18.470.140.02101.120.049.36 6.010.56 1.82 0.006.170.030.0015.978.03 3222.8339 .08 60% outward 0.69 0.22 34.32 34.62 3.34 9.73 0.01 18.250.160.02100.680.049.14 6.190.61 1.84 0.006.150.030.0015.978.02 3223.0340 .36 90% outward 0.69 0.20 37.78 30.75 3.34 10.610.02 18.090.120.02100.930.039.93 5.420.60 1.98 0.006.010.020.0015.988.02 3224.7735 .32 rim 0.69 0.26 36.52 31.18 4.06 11.220.03 17.620.090.02101.000.049.66 5.530.73 2.11 0.015.890.020.0015.978.03 3226.3236.42 D22-3-sp4 ctr lg. round in glass 0.69 0.25 27.52 42.52 3.14 9.82 0.03 17.630.060.02100.980.047.53 7.810.58 1.91 0.016.100.010.0015.978.03 3223.8050.89 15% outward 0.69 0.25 27.63 42.35 3.38 9.78 0.04 17.690.090.02101.230.047.54 7.750.63 1.89 0.016.110.020.0015.978.03 3223.6750 .69 30% outward 0.69 0.23 28.48 41.23 3.32 9.87 0.03 17.660.060.02100.900.047.77 7.540.62 1.91 0.016.090.010.0015.978.03 3223.8849 .27 45% outward 0.69 0.26 31.69 37.91 3.16 10.290.04 17.760.110.04101.260.048.51 6.830.58 1.96 0.016.030.020.0115.968.03 3224.5344 .52 60% outward 0.69 0.21 36.54 31.78 3.93 10.500.05 18.050.130.05101.240.049.62 5.610.70 1.96 0.016.010.020.0115.988.02 3224.6236 .85 75% outward 0.69 0.20 36.65 32.51 3.20 9.83 0.04 18.480.150.03101.090.039.64 5.730.57 1.83 0.016.140.030.0115.988.02 3222.9937 .30

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197Table B-3. Continued. Sample # Description Mg* TiO2 Al2O3 Cr2O3Fe2O3FeOMnOMgONiOCaOTotalTi Al CrFe3+Fe2+MnMgNiCaTETOCTOFE*CR* 90% outward 0.69 0.23 36.41 32.82 2.98 9.77 0.04 18.43 0.19 0.06 100.930.04 9.60 5.800.53 1.83 0.016.140.030.01 15.978.03 3222.9337.68 rim 0.69 0.28 36.45 30.67 4.10 11.230.05 17.46 0.09 0.03 100.360.05 9.70 5.470.74 2.12 0.015.880.020.01 15.968.03 3226.5236.08 D22-3-sp5 sm. round in oliv. 0.69 0.57 37.35 28.01 5.17 11.040.04 17.78 0.12 0.02 100.100.10 9.91 4.980.93 2.08 0.015.960.020.00 15.928.08 3225.8333.47 D22-3-sp6 euh. within glass 0.69 0.51 32.26 32.97 5.25 11.710.03 16.70 0.08 0.01 99.52 0.09 8.82 6.040.98 2.27 0.015.770.010.00 15.938.07 3228.2340.67 D27-5-sp1 ctr euh. att. to olivine 0.69 0.68 31.93 33.18 5.47 12.220.21 16.45 0.01 0.08 100.240.12 8.70 6.061.01 2.36 0.045.670.000.02 15.908.09 3229.4341.07 50% outward 0.69 0.54 31.83 33.40 5.36 12.160.18 16.32 0.07 0.08 99.94 0.09 8.70 6.131.00 2.36 0.045.640.010.02 15.928.07 3229.4841.31 rim 0.69 0.57 31.68 32.40 6.25 11.260.20 16.75 0.05 0.09 99.25 0.10 8.69 5.961.17 2.19 0.045.810.010.02 15.928.07 3227.3840.69 D27-5-sp2 euh. within sm. oliv. 0.69 0.49 31.29 34.05 5.33 11.870.18 16.45 0.02 0.02 99.70 0.09 8.58 6.270.99 2.31 0.045.710.000.00 15.938.06 3228.8342.20 D27-5 -sp3 euh. within sm. oliv. 0.69 0.54 30.17 32.33 7.07 10.220.18 16.92 0.10 0.03 97.56 0.10 8.43 6.061.34 2.03 0.045.980.020.01 15.928.07 3225.3141.82 D27-5-sp4 ctr euh att. oliv. 0.69 0.58 31.05 33.48 5.62 11.540.19 16.42 0.02 0.20 99.10 0.10 8.56 6.191.05 2.26 0.045.730.000.05 15.918.08 3228.2941.97 D27-5-sp5 ctr euh.att. oliv. 0.69 0.52 31.27 34.11 5.33 12.230.20 16.17 0.10 0.11 100.030.09 8.57 6.270.99 2.38 0.045.600.020.03 15.928.07 3229.7942.25 D27-5-sp6 ctr euh. att. oliv. 0.69 0.53 30.87 32.97 5.97 11.790.15 16.16 0.05 0.14 98.63 0.09 8.57 6.141.13 2.32 0.035.670.010.04 15.928.07 3229.0541.74 D27-5-sp7 ctr med. att. oliv. 0.69 0.57 31.26 33.23 5.94 11.590.14 16.55 0.09 0.09 99.47 0.10 8.59 6.121.11 2.26 0.035.750.020.02 15.928.07 3228.2141.63 rim 0.69 0.56 31.57 32.57 6.17 11.340.13 16.60 0.05 0.25 99.24 0.10 8.67 6.001.15 2.21 0.035.760.010.06 15.928.07 3227.7140.90 D27-5-sp8 ctr. euh. in glass 0.69 0.52 30.99 33.52 5.82 11.500.11 16.41 0.10 0.22 99.18 0.09 8.54 6.201.09 2.25 0.025.720.020.06 15.938.07 3228.2242.05 D27-5-sp9 euh. within oliv. 0.69 0.50 31.00 33.17 5.63 11.690.10 16.32 0.05 0.02 98.48 0.09 8.60 6.171.06 2.30 0.025.730.010.01 15.938.06 3228.6741.78 2384-9-sp1 euh. att. oliv. 0.71 0.30 43.45 22.40 4.27 10.130.10 18.63 0.16 0.06 99.50 0.05 11.2 6 3.900.75 1.86 0.026.110.030.01 15.968.03 3223.3725.70 2384-9-sp2 euh. att. oliv. 0.71 0.40 35.34 30.64 5.11 10.550.04 17.72 0.12 0.09 100.010.07 9.45 5.500.93 2.00 0.015.990.020.02 15.958.05 3225.0436.77 2384-9-sp3 euh. att. oliv. 0.71 0.97 35.44 29.56 5.02 10.820.05 17.80 0.19 0.13 99.98 0.17 9.47 5.300.91 2.05 0.016.020.030.03 15.858.14 3225.4435.88 2384-9-sp4 euh. att. oliv. 0.71 0.40 35.24 30.40 5.24 10.210.08 17.81 0.12 0.10 99.61 0.07 9.45 5.470.96 1.94 0.026.040.020.02 15.958.05 3224.3436.66 2384-1-sp1 euh. att. oliv. 0.71 0.25 37.60 28.65 4.49 9.70 0.09 18.25 0.17 0.02 99.22 0.04 10.0 0 5.110.81 1.83 0.026.140.030.00 15.978.02 3222.9733.82 2384-1-sp2 ctr. euh. att. oliv. 0.71 0.31 38.55 27.87 4.53 9.70 0.12 18.54 0.03 0.03 99.67 0.05 10.1 7 4.930.81 1.81 0.026.180.010.01 15.968.03 3222.6932.66 rim 0.71 0.30 38.48 27.74 4.43 9.92 0.08 18.31 0.09 0.03 99.37 0.05 10.1 9 4.930.80 1.86 0.026.130.020.01 15.968.03 3223.3132.59

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198Table B-3. Continued. Sample # Description Mg* TiO2 Al2O3 Cr2O 3 Fe2O3FeOMnOMgONiOCaOTotalTi Al CrFe3+Fe2+MnMgNiCaTETOCTOFE*CR* 2384-11sp1 euh. within glass 0.67 0.98 27.29 36.29 6.35 13.330.08 15.39 0.02 0.11 99.85 0.187.64 6.821.21 2.65 0.025.450.00 0.0 3 15.848.15 32 32.7147.15 50% outward 0.67 0.83 28.02 35.34 6.43 13.100.06 15.42 0.01 0.14 99.34 0.157.85 6.641.23 2.61 0.015.470.00 0.0 4 15.878.12 32 32.2845.83 rim 0.67 0.72 28.81 34.75 5.76 13.280.03 15.17 0.02 0.19 98.73 0.138.10 6.561.10 2.65 0.015.400.00 0.0 5 15.898.10 32 32.9444.72 2384-4asp1 in oliv. 0.61 0.77 21.938 40.01 2.189417.7 0.33 14.3 0 0 97.12 0.156.55 2 8.01 9 0.42 3.76 0.1 5.4 0 0 15.149.22 32 41.1255.03 2384-4asp2 in oliv. 0.61 0.685 21.424 39.9952.199 17.8 0.21 14 0 0 96.17 0.136.47 2 8.10 9 0.42 3.82 0 5.4 0 0 15.149.23 32 41.6555.61 2384-4asp3 in oliv. 0.61 0.69 21.395 40.1532.229718.1 0.19 13.9 0 0 96.51 0.136.44 9 8.12 2 0.43 3.87 0 5.3 0 0.0 1 15.139.23 32 42.1155.74 2384-4asp4 in oliv. Clot 0.61 0.513 33.341 30.5812.076116.8 0.27 15.7 0 0 99.17 0.099.24 5.68 7 0.37 3.31 0.1 5.5 0 0.0 1 15.398.88 32 37.5538.10 Notes: Notes: Mg = 100 Mg/(Mg + Fe2+) of the host glass. Oxides expressed in wt. %. FE* = Fe2+/(Fe2+ + Mg). CR* = Cr/(Cr + Al).

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APPENDIX C MAJOR ELEMENT COMPOSITIONS OF THE SIQUEIROS SAMPLES

PAGE 213

200Table C-1. ARL, JEOL, and DCP electron microprobe major el ement analyses of basalts from the Siqueiros transform. Sample Location Depth (m) SiO2TiO2Al2O3FeO Fe2O3MnOMgOCaO Na2O K2O P2O5Total Mg# Na8.0 Fe8.0 Weight percent by microprobe* 2375-1 B ND 50.231.4715.018.97 1.11 0.17 7. 93 12.13 2.53 0.08 0.11 99.46 61.14 2.51 9.88 2375-2 B ND 50.781.7614.0110.071.24 0.2 7.13 11. 76 2.67 0.1 0.14 99.54 55.75 2.47 10.06 2375-4 B 2992 51.031.6314.269.37 1.16 0.18 7.56 11. 86 2.57 0.1 0.13 99.55 58.97 2.48 9.86 2375-6 B 2985 50.251.4115.088.92 1.10 0.18 8.09 12. 04 2.45 0.08 0.14 99.46 61.76 2.47 10.02 2375-7 B 2968 50.291.4115.578.76 1.08 0.2 8.03 12. 12 2.62 0.07 0.1 100.0862.01 2.62 9.77 2375-9 B 2955 50.7 1.5315.1 9.01 1.11 0.19 7.61 11. 82 2.72 0.11 0.13 99.86 60.06 2.65 9.53 2376-1 B 3073 50.031.4215.598.50 1.05 0.15 8.53 11. 88 2.37 0.11 0.12 99.48 64.13 2.45 10.03 2376-2 B ND 50.591.6314.749.14 1.13 0.18 7.83 11. 81 2.38 0.13 0.19 99.46 60.4 2.35 9.96 2376-3 B 2023 50.121.5 15.488.76 1.08 0.16 7.85 11. 88 2.6 0.13 0.15 99.55 61.48 2.57 9.55 2376-4 B 3015 50.231.4415.488.45 1.04 0.16 8.4 11. 91 2.42 0.11 0.13 99.53 63.9 2.49 9.85 2376-7 B 3092 49.831.3915.978.21 1.01 0.2 8.4 11. 6 2.58 0.11 0.12 99.27 64.56 2.64 9.58 2376-8 B 3037 49.561.7 15.939.16 1.13 0.19 8.02 11. 27 2.69 0.1 0.17 99.76 60.91 2.7 10.2 2376-12 B ND 49.891.4115.438.72 1.08 0.19 8.35 12. 15 2.43 0.11 0.13 99.6 63.02 2.49 10.08 2377-1 B 31701 50.091.3115.698.60 1.06 0.2 8.35 12.2 2.46 0.08 0.1 99.99 63.36 2.52 9.95 2377-2 B 3119 50.251.8114.829.53 1.18 0.19 7.55 11. 42 2.81 0.12 0.19 99.54 58.51 2.72 10.03 2377-3 B 3083 50.011.6715.379.06 1.12 0.2 7.72 11. 52 2.69 0.09 0.15 99.44 60.26 2.64 9.73 2377-4 B 3028 49.841.9 15.349.46 1.17 0.19 7.63 11. 1 2.76 0.11 0.18 99.51 58.95 2.68 10.06 2377-5 B 3090 50.282.0614.7110.021.24 0.2 7.18 10. 76 2.97 0.16 0.25 99.5 56.08 2.79 10.08 2377-6 B 3011 50.552.1614.0910.111.25 0.18 6.99 11. 27 2.81 0.15 0.21 99.46 55.16 2.57 9.91 2377-7 B 3051 49.851.7715.199.32 1.15 0.18 8. 03 11.43 2.66 0.13 0.19 99.6 60.53 2.67 10.4 2377-8 B 3087 49.951.7 15.159.23 1.14 0.17 8. 02 11.53 2.61 0.12 0.17 99.5 60.75 2.61 10.28 2377-10 B 3085 50.141.9515.029.69 1.20 0.19 7. 51 10.96 2.68 0.15 0.21 99.39 57.99 2.58 10.16 2377-11 B 3033 49.932.1514.8310.181.26 0.23 7. 14 10.79 2.76 0.14 0.25 99.48 55.53 2.56 10.2 2378-1 C 2303 50.731.5814.139.78 1.21 0.19 7. 32 11.92 2.8 0.09 0.13 99.58 57.12 2.66 10.01 2378-2 C 2223 50.191.3515.628.53 1.05 0.19 8. 43 12.17 2.48 0.1 0.11 100.0863.76 2.55 9.97 2378-3 C 2234 50.561.3314.698.92 1.10 0.17 8 12.22 2.63 0.08 0.1 99.52 61.51 2.63 9.91 2378-5 C 2246 50.551.3314.598.93 1.10 0.18 8 12.34 2.64 0.08 0.11 99.58 61.48 2.64 9.92 2378-7 C 2270 50.651.3914.799.14 1.13 0.2 7.76 12 2.67 0.07 0.11 99.75 60.17 2.63 9.87 2378-8 C 2287 50.431.1315.067.93 0.98 0.14 8. 28 12.97 2.66 0.04 0.08 99.45 65.03 2.7 9.14

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201Table C-1. Continued. Sample Location Depth (m) SiO2TiO2Al2O3FeO Fe2O3MnOMgOCaO Na2OK2O P2O5Total Mg# Na8.0 Fe8.0 Weight percent by microprobe* 2380-4 B 3178 50.761.7214.829.32 1.15 0.22 7. 56 11.39 2.44 0.12 0.16 99.5 59.08 2.35 9.82 2380-11 B 3079 49.841.9715.149.90 1.22 0.25 7. 45 10.9 2.66 0.13 0.22 99.5 57.26 2.55 10.31 2381-11 B-C 2686 50.571.1915.068.93 1.10 0.23 7.76 12.33 2.64 0.03 0. 07 99.75 60.74 2.59 9.63 2381-11WR B-C 2686 50.571.1915.068.93 1.10 0.23 7. 76 12.33 2.64 0.03 0.07 99.75 60.74 2.59 9.63 2381-14A B-C 2485 50.411.0715.5 7.95 0.98 0. 26 8.43 12.66 2.54 0.03 0. 07 99.76 65.38 2.61 9.32 2382-7 B 2474 50.551.7 14.539.63 1.19 0.21 7. 39 11.54 2.76 0.11 0.18 99.61 57.74 2.63 9.93 2383-2 A 3735 51.041.4914.419.37 1.16 0.24 7.52 11.74 2.6 0.08 0. 14 99.64 58.83 2.5 9.81 2383-6 A 3661 50.541.1 15.348.04 0.99 0.18 8.89 12.54 2.2 0.03 0. 08 99.83 66.32 2.32 9.89 2384-1 A-B 3884 48.880.9217.5 6.99 0.86 0.16 9.6 12.22 2.4 ND 0.06 99.46 70.96 2.55 9.37 2384-2 A-B 3841 49.120.9617.387.25 0.89 0.16 9.54 12.17 2.4 ND 0.04 99.78 70.1 2.55 9.6 2384-3 A-B 3751 48.950.9517.357.18 0.89 0.17 10.1212.09 2.33 ND 0.06 99.97 71.5 2.48 9.97 2384-6 A-B 3707 49.2 0.9517.147.43 0.92 0.16 9.57 12.38 2.47 ND 0.05 99.98 69.62 2.63 9.84 2384-7A A-B 3646 49.580.8817.866.99 0.86 0.14 9.9 12.23 2.4 ND 0.03 100.5971.59 2.55 9.6 2384-7B A-B 3646 49.1 0.9 17.697.00 0.86 0.12 9.93 12.23 2.43 ND 0.06 100.0471.63 2.59 9.64 2384-8 A-B 3648 49.781.0217.2 7.34 0.90 0.1 9.85 11.93 2.51 ND 0.06 100.3870.51 2.66 9.95 2384-9 A-B 3623 49.021.0117.077.25 0.89 0.16 9.73 11.86 2.45 0.01 0. 07 99.4 70.48 2.61 9.76 2384-10 A-B 3593 49.691.1316.897.40 0.91 0.14 9.59 11.87 2.52 ND 0.06 99.9 69.77 2.67 9.81 2384-11 A-B 3558 49.7 1.1716.167.83 0.97 0.12 8.79 11.83 2.47 ND 0.08 99.99 66.65 2.58 9.56 2384-12 A-B 3525 49.821.1816.897.53 0.93 0.18 9.11 11.96 2.68 ND 0.07 100.0468.28 2.82 9.54 2385-2 C 2352 50.291.1215.068.23 1.01 0.2 8.21 12.34 2.65 0.01 0. 06 99.02 63.99 2.69 9.38 2385-3A C 2333 50.431.3614.978.46 1.04 0.22 7.8 11.95 2.79 0.04 0. 1 98.98 62.14 2.75 9.16 2385-6T C 2347 50.861.2 14.958.67 1.07 0.2 7.91 11.91 2.72 0.03 0. 1 99.45 61.9 2.71 9.52 2386-5 D 2176 50.761.0915.188.41 1.04 0.2 8. 23 12.52 2.44 0.02 0.09 99.81 63.54 2.48 9.61 2386-7 D 2058 50.651.3114.649.69 1.20 0.27 7.52 11.88 2.61 0.04 0.08 99.71 58 2.51 10.17 2387-2 (3) B-C 3206 51.041.5214.938.38 1.03 0. 16 7.56 11.23 2.94 0.08 0. 1 98.71 61.6 2.85 8.76 2387-5 B-C 3150 50.251.5115.249.06 1.12 0.26 7.79 11.68 2.81 0.03 0. 12 99.69 60.48 2.77 9.82 2387-6 B-C 3057 50.551.5114.619.50 1.17 0.2 7.44 11.73 2.86 0.03 0. 14 99.55 58.25 2.75 9.85 2388-3A A-B 3909 50.311.8214.519.84 1.21 0.27 7.36 11.45 2.55 0.08 0. 18 99.41 57.12 2.42 10.12 2388-10 A-B 3057 50.581.1 15.348.36 1.03 0.17 8.44 12.63 2.29 0.01 0. 07 99.9 64.25 2.36 9.79 2389-1 A 3714 50.891.7614.3610.011.23 0.21 7. 35 11.26 2.51 0.04 0.17 99.63 56.66 2.37 10.3 2390-1 W-RTI 3004 48.972.4715.4 9.36 1.15 0.18 6.37 10.37 3.19 0.73 0. 48 98.48 54.78 2.76 8.16

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202Table C-1. Continued. Sample Location Depth (m) SiO2TiO2Al2O3FeO Fe2O3MnOMgOCaO Na2O K2OP2O5Total Mg# Na8.0 Fe8.0 Weight percent by microprobe* 2390-9 W-RTI 2930 50.012.8412.8712.621.56 0.27 5.54 9.47 3.01 0.3 0. 32 98.58 43.87 2.26 10.4 2391-1 A-B 3721 50.631.4214.889.03 1.11 0.27 7.95 11.99 2.39 0.06 0. 12 99.68 61.06 2.38 9.97 D1-3 B 3000 50.581.4215.218.90 1.10 0.25 8. 04 12.18 2.6 0.07 0.09 100.2861.66 2.6 9.94 D1-5 B 3000 50.781.4315.248.80 1.09 0.24 8. 07 12.21 2.54 0.07 0.1 100.4162.01 2.56 9.86 D4-2 B 3000 51.421.6614.439.69 1.20 0.29 7. 25 11.57 2.67 0.08 0.11 100.2157.11 2.51 9.81 D4-4 B 3000 51.221.5314.759.05 1.12 0.25 7. 8 11.74 2.68 0.09 0.1 100.1560.53 2.64 9.82 D4-6 B 3000 51.071.6 14.589.46 1.17 0.26 7. 49 11.57 2.76 0.07 0.14 99.99 58.5 2.65 9.87 D6-1 B 3200 50.531.4115.618.75 1.08 0.24 8. 03 11.95 2.82 0.07 0.09 100.4162.04 2.82 9.76 D13-1 B 2900 50.621.7814.7 9.60 1.18 0.3 7. 41 11.43 2.84 0.1 0.12 99.9 57.87 2.72 9.93 D15-1 B 2900 50.531.7814.739.71 1.20 0.27 7. 33 11.4 2.81 0.11 0.12 99.81 57.33 2.67 9.94 D17-10 A-B 3000 50.2 1.6515.319.15 1.13 0.23 8.04 11.25 2.61 0.11 0.13 99.65 61 2.61 10.22 D17-11 A-B 3000 50.4 1.3 16.028.42 1.04 0.17 8.26 11.82 2.69 0.11 0. 15 100.2 63.6 2.74 9.65 D18-3 B 2800 50.851.8414.2810.261.27 0.2 6.72 11. 06 3.05 0.2 0.2 99.74 53.83 2.74 9.69 D18-4 B 2800 50.771.8914.2310.371.28 0.21 6.64 11. 02 3.08 0.19 0.19 99.67 53.27 2.75 9.69 D19-2 B 2650 50.761.8314.2610.131.25 0.2 7.02 11. 02 2.99 0.13 0.17 99.58 55.22 2.76 9.99 D19-9 B 2650 51.451.8214.6510.121.25 0.2 6.9 11. 14 3.18 0.11 0.18 100.8 54.84 2.92 9.79 D20-1 A-B 3100 48.810.9617.317.13 0.88 0.1 10.0112.08 2.39 ND 0.06 99.61 71.43 2.54 9.83 D20-2 A-B 3100 49.420.9617.447.23 0.89 0. 12 10.0612 2.44 ND 0.06 100.3371.25 2.59 9.98 D20-3 A-B 3100 49.450.9417.577.19 0.89 0.13 9.86 12.21 2.43 ND 0.07 100.4470.94 2.59 9.79 D20-4 A-B 3100 49.2 0.9417.847.19 0.89 0.14 9.18 12.55 2.46 ND 0.03 100.1469.44 2.6 9.23 D20-6 A-B 3100 49.080.9417.327.15 0.88 0.12 9.87 12.13 2.42 ND 0.03 99.65 71.09 2.58 9.75 D20-7 A-B 3100 49.060.9417.417.15 0.88 0.12 9.76 12.11 2.41 ND 0.08 99.78 70.86 2.56 9.67 D20-8 A-B 3100 49.040.9217.377.11 0.88 0.09 9.81 12.06 2.46 ND 0.07 99.69 71.07 2.62 9.67 D20-12 A-B 3100 50.751.8 14.459.97 1.23 0.21 6.96 11.48 2.88 0.09 0. 14 99.59 55.41 2.64 9.72 D20-13 A-B 3100 49.160.9617.427.14 0.88 0.1 10.2212.02 2.44 ND 0.05 100.1171.83 2.59 9.99 D20-14 A-B 3100 49.560.9 17.457.11 0.88 0.1 10.1112.09 2.41 ND 0.06 100.3971.68 2.56 9.88 D20-15 A-B 3100 49.230.9417.5 7.16 0.88 0.11 9.89 12.09 2.45 ND 0.07 100.2 71.11 2.6 9.78 D20-16 A-B 3100 49.310.9417.4 7.08 0.87 0.12 10.0812.08 2.44 ND 0.08 100.1171.7 2.59 9.83

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203Table C-1. Continued. Sample Location Depth (m) SiO2TiO2Al2O3FeOFe2O3MnOMgOCaO Na2O K2OP2O5Total Mg# Na8.0Fe8.0 Weight percent by microprobe* D20-18 A-B 3100 49.780.9 17.867.160.88 0.12 9.96 11.93 2.4 ND 0.08 100.7971.22 2.55 9.84 D20-19 A-B 3100 49.440.9217.537.110.88 0.12 10.0212.22 2.43 ND 0.07 100.4571.5 2.58 9.82 D20-20 A-B 3100 48.990.9517.217.130.88 0. 05 10.4712.1 2.47 ND ND 99.96 72.34 2.61 10.14 D20-21 A-B 3100 49.3 0.9617.3 7.180.89 0.16 10.1412.13 2.46 ND 0.01 100.2371.54 2.61 9.98 D20-30 A-B 3100 49.090.9517.437.200.89 0.11 9.89 12.03 2.44 ND 0.06 99.96 70.98 2.59 9.83 D20-31 A-B 3100 49.660.9217.7 7.080.87 0. 12 10.1612 2.43 ND 0.07 100.7171.86 2.58 9.89 D20-33 A-B 3100 48.730.9817.337.060.87 0.14 9.92 12.22 2.53 ND 0.1 99.59 71.45 2.68 9.69 D20-40 A-B 3100 49.2 0.9417.557.130.88 0.16 10.1112.03 2.39 ND 0.1 100.2 71.63 2.54 9.9 D21-1 A-B 3800 48.761.2516.749.261.14 0.18 8.61 11.19 2.48 0.05 0. 1 99.6 62.34 2.57 10.97 D22-2 A-B 3800 49.691.1316.657.500.92 0.11 9.46 11.82 2.53 0.01 0. 08 99.77 69.2 2.69 9.81 D22-3 A-B 3800 49.951.1 16.847.560.93 0.15 9.47 11.88 2.55 ND 0.1 100.4 69.04 2.71 9.89 D22-4 A-B 3800 49.761.1116.867.670.95 0.13 9.21 11.81 2.59 ND 0.08 100.0268.14 2.73 9.78 D23-2 A-B 3800 50.061.1316.737.600.94 0.14 9.42 11.88 2.52 0.02 0. 07 100.4 68.83 2.67 9.89 D25-6 C 2700 51.861.6914.569.591.18 0.15 7. 37 11.6 2.39 0.12 0.14 100.5 57.79 2.25 9.86 D26-4 C 2500 50.861.5914.7 9.591.18 0.2 7.38 11.45 2.86 0.14 0. 13 99.91 57.8 2.73 9.88 D26-6 C 2500 51.011.7 14.379.841.21 0.19 7. 04 11.44 2.92 0.13 0.14 99.8 56.03 2.7 9.68 D27-5 C-D 2500 49.961.1416.54 7.700.95 0.14 9.41 11.94 2.54 0.02 0.06 100.3 68.52 2.69 9.99 D30-1 E-RTI 2800 50.581.7714.479.651.19 0.21 7.35 11.47 2.9 0.09 0. 14 99.65 57.56 2.77 9.9 D32-1 C 2300 51.021.3515.439.191.13 0.19 7.69 12.06 2.99 0.03 0. 07 101 59.84 2.93 9.83 D32-3 C 2300 50.671.5514.769.411.16 0.19 7. 34 11.91 2.88 0.08 0.13 99.91 58.13 2.74 9.63 D34-2 C-D 2400 49.861.0216.28 7.790.96 0.12 9.12 12.96 2.18 0.06 0.05 100.3 67.59 2.32 9.83 D35-3 A 3100 50.011.7215.289.111.12 0.19 7.77 11. 54 2.59 0.1 0.17 99.44 60.3 2.54 9.84 D35-4 A 3100 49.811.7215.599.031.11 0.18 7.88 11. 56 2.63 0.1 0.16 99.62 60.85 2.61 9.89 D36-3 A 3200 50.561.2515.258.481.05 0.18 8.39 12. 36 2.26 0.07 0.09 99.8 63.79 2.32 9.86 D36-4 A 3200 50.551.2515.278.511.05 0.17 8.42 12. 38 2.25 0.05 0.12 99.87 63.8 2.31 9.93 D37-2 A 3000 50.4 1.3915.158.711.07 0.18 8.31 12. 22 2.43 0.04 0.13 99.9 62.94 2.48 10.04 D38-1 A 3500 50.211.1715.538.591.06 0.18 8.78 12. 42 2.24 0.03 0.08 100.1 64.55 2.35 10.39 D38-2 A 3500 50.691.6814.789.321.15 0.21 7. 66 11.52 2.57 0.1 0.15 99.65 59.42 2.5 9.93 D39-1 W-RTI 3000 50.372.2914.1110.671.32 0. 22 7 10.31 2.88 0.24 0. 28 99.49 53.89 2.65 10.55 D43-2 W-RTI 3000 50.571.3814.998.801.09 0.22 7.99 12.22 2.61 0.07 0. 09 99.87 61.78 2.61 9.77 D44-1 DW 2100 50.691.2215 8.691.07 ND 7. 99 12.72 2.47 0.05 0.07 100 62.09 2.47 9.64

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204Table C-1. Continued. Sample Location Depth (m) SiO2TiO2Al2O3FeO Fe2O3MnOMgOCaO Na2O K2OP2O5Total Mg# Na8.0Fe8.0 Weight percent by microprobe* 2388-2 A-B 3500 48.9 1.2817.138.79 1.08 0.19 8.77 11.16 2.8 0.03 0. 08 99.89 63.97 2.91 10.61 2388-5 A-B 3775 50.511.8614.219.41 1.16 0.18 7.45 11.57 2.53 0.22 0. 17 ND 58.51 2.41 9.76 2388-13 A-B 3072 50.571.4614.688.88 1.10 0.2 7.95 12.1 2.63 0.04 0. 09 ND 61.44 2.62 9.81 RC-40 E-RTI 2800 50.531.4814.788.66 1.07 0. 17 7.94 12.2 2.68 0.07 0.13 ND 62.02 2.67 9.55 Rc -41 E-RTI 2800 50.381.4515.158.75 1.08 0. 19 8.02 12.05 2.62 0.05 0.1 ND 62.01 2.62 9.74 RC -42 E-RTI 2800 50.241.4815.428.51 1.05 0. 19 8.03 12.03 2.76 0.05 0.12 ND 62.67 2.76 9.5 Weight percent by microprobe for picritic basalts and picrites 2384-6 A-B 3707 48.430.8217.356.95 0.86 0.1 12.4111.48 2.25 0.03 0. 06 100.7376.10 2.72 8.99 2384-1 A-B 3884 47.050.8916.117.16 0.88 0.15 12.7811.13 2.18 0.0390. 07798.45 76.00 2.67 9.24 2384-3 A-B 3751 47.9 0.8516.187.13 0.88 0.1 14.2410.64 2.16 0.03 0. 07 100.1878.10 2.75 9.32 D20-1 A-B 3100 47.380.8 15.387.17 0.88 0.1 16.489.98 1.99 0.03 0. 06 100.2680.40 2.72 9.54 2384-2 A-B 3841 46.930.7113.157.37 0.91 0.12 20.579.14 1.7 0.03 0. 05 100.6883.20 2.70 10.07 D20-15 A-B 3100 46.350.7113.967.21 0.89 0.11 21.1 8.64 1.77 0.02 0. 05 100.8183.00 2.80 9.95 Weight percent by DCP^ 2384-1 A-B 3884 47.050.8916.117.17 0.88 0.15 12.7811.13 2.18 0.04 0. 08 99.25 76.06 2.67 9.24 2384-3 A-B 3751 48.160.9717.077.15 0.88 0.15 10.4911.89 2.36 0.03 0. 08 100.04 72.33 2.70 9.04 2384-7B A-B 3646 48.140.9017.276.96 0.86 0.15 10.6312.08 2.45 0.04 0. 08 100.33 73.11 2.80 8.86 2384-8 A-B 3648 48.301.0416.877.32 0.90 0.15 10.3411.73 2.46 0.04 0. 08 100.05 71.56 2.80 9.20 2390-5 W-RTI 3110 48.482.0516.338.70 1.07 0.16 7.74 10.35 3.03 0.68 0. 35 99.92 61.32 2.97 8.42 D20-5 A-B 3100 48.050.9517.117.11 0.88 0.14 10.6311.71 2.37 0.03 0. 08 99.85 72.70 2.72 9.01 D20-13 A-B 3100 48.220.9717.056.30 0.78 0.14 10.6011.89 2.37 0.03 0. 08 99.14 74.98 2.72 8.20 D20-15 A-B 3100 50.950.9918.067.15 0.88 0.02 11.2412.35 2.55 0.04 0. 09 105.25 73.69 2.94 9.10 RC-41 E-RTI 2800 49.831.4615.448.81 1.09 0.18 8.00 12.09 2.68 0.08 0. 12 100.75 61.80 2.68 8.81 Locations: A = spreading center A, A-B = fault separating spreading centers A and B, B = spreading center B, B-C = fault separa ting spreading centers B and C, C = spreading center C, C-D = fault separating spreading center C and trough D, D = trough D, WRTI = western ridge transform in tersection, ERTI = eastern ridge transform intersection. *Microprobe analysis was completed on natural glass samples at the US Geological Survey in Denve r using an ARL-SEMQ microprobe and JEOL microprobe. Major element analysis of the picritic basalts and picrites de termined by electron microprobe analysis of fused glasses + phenocrysts. ^ DCP analysis was completed on phenocrysts-free samples at the La mont-Doherty Earth Observatory. ND = not detected; Mg# = Mg/(Mg + Fe2+); Fe2+ is assumed to be 0.9 Fe total. All probe analyses were normalized to standard glasses VG-A99 and JdF-D2 which were run conc urrently with the Siqueiros glasses. Na8.0 = Na2O contents normalized to 8.0 wt. % MgO. Fe8.0 = FeO contents normalized to 8.0 wt. % MgO.

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205Table C-2. Siqueiros glass major element analysis. Sample # Loc. Depth (m) SiO2 TiO2Al2O3FeOtMnOMgO*CaONa2OK2O P2O5**CR2O3TotalMg #Na8.0Fe8.0H2OS CL Weight percent by microprobe^ 2390-9 W-RTI 2930 50.9 2.8513.1214.20.255.48 9.862. 9 0.22 0.32 0.01 100.143.27 2.2710.090.3910.2020.058 2390-5 W-RTI 3010 49.872.5 15.7710.60.196.40 10.53.0 40.71 0.50 0.04 100.154.47 2.647.810.6490.1490.035 2390-3B/ar2 W-RTI 2934 50.282.4815.43 10.80.216.42 10.653.040.65 0.48 0. 03 100.454.192.657.980.6360.1560.032 2390-3B/ar1 W-RTI 2934 50.212.4515.52 10.80.216.46 10.593 0.65 0.47 0.02 100.354.222.628.050.6340.1560.033 2390-4 W-RTI 2938 50.2 2.4415.5 10.80.196.51 10.692. 990.64 0.47 0.03 100.454.41 2.628.120.6320.1480.032 D39-1 W-RTI 3000 50.832.2713.9512 0.246.96 10.372. 8 0.21 0.28 0.03 99.953.42 2.549.690.3320.1660.067 2377-6/ar1 B 3011 50.762.1214.3311.40.227.06 11.32.6 70.13 0.23 0.04 100.354.98 2.449.280.2900.1490.018 2377-6/ar2 B 3011 50.772.0914.3811.40.227.09 11.342. 670.14 0.22 0.06 100.355.24 2.449.260.2950.1530.018 2377-5 B 2090 50.742.0615.1111.10.197.17 10.82.8 0.13 0.24 0.04 100.356.22 2.599.050.3410.1390.022 2389-1/ar1 A 3714 50.991.7114.2810.90.227.41 11.552. 520.08 0.15 0.03 99.857.35 2.379.190.1460.1490.004 2387-5/p1/ar1 B-C 3150 50.851.5914.6710.40.187.43 12. 072.9 0.05 0.12 0.05 100.258. 692.768.720.1350.1410.004 2387-5/p1/ar2 B-C 3150 50.691.5614.6310.50.227.43 12. 212.910.06 0.12 0.03 100.358. 452.778.810.1360.1300.003 2380-12 BE 3069 50.031.9815.1 11 0.2 7.45 11.12.660 .13 0.23 0.04 99.957.28 2.529.300.3080.1420.019 2389-1/ar2 A 3714 51.2 1.7214.3911 0.197.47 11.672. 530.08 0.15 0.05 100.457.27 2.409.360.1470.1510.004 2387-5/p2/ar1 B-C 3150 50.661.5914.53 10.30.197.49 12.152.890.06 0.11 0. 03 100.059.052.768.700.1300.1320.002 2387-5/p2/ar2 B-C 3150 50.551.5314.61 10.30.2 7.56 12.062.9 0.07 0.12 0. 04 99.959.322.798.760.1250.1420.003 2377-10 B 3085 49.981.9415.1911 0.2 7.60 11.062.61 0.13 0.21 0.02 99.957.86 2.519.440.2890.1420.018 2389-5 A 3552 51.191.5614.2811 0.217.68 12.192.33 0.07 0.12 0.03 100.757.93 2.259.590.1330.1420.002 2378-7 C 2278 50.8 1.3814.7610.40.217.77 12.162.7 0.07 0.10 0.04 100.359.77 2.649.070.1410.1220.009 2385-3B C 2332 51.151.3414.7 10.20.167.76 12.162.83 0.07 0.11 0.03 100.560.11 2.778.910.1180.1320.003 2380-9 B 3135 50.041.8 15.3810.70.197.82 11.422.56 0.12 0.19 0.06 100.259.25 2.529.390.2630.1360.014 2388-13 A-B 3072 50.7 1.4614.7810 0.177.93 12.392.54 0.06 0.11 0.05 100.261.05 2.528.950.1420.1330.007 2389-4 A 3555 51.081.4814.4510.70.227.93 12.042.34 0.07 0.12 0.05 100.459.46 2.329.570.1270.1380.001 2376-3/p2 B 2023 50.2 1.5 15.389.930.2 7.98 11.922. 510.11 0.17 0.05 99.961.40 2.508.910.2130.1210.010 D17-9 A-B 3000 50.391.6715.1810.40.248.01 11.592. 510.10 0.17 0.04 100.360.37 2.519.380.2140.1330.010 D35-4 A 3100 50.281.7615.6310.40.2 8.03 11.612.62 0.11 0.19 0.04 100.860.44 2.639.410.2380.1380.007 2376-3/p1/ar1 B 2023 50.261.4815.389. 810.148.04 11.992.540.11 0.16 0.04 99.961.882.558.880.2130.1350.011 2378-3 C 2234 50.511.2814.919.960. 248.04 12.442.580.07 0.11 0.05 100. 161.522.599.010.1250.1290.010 2378-2/p2 C 2223 50.751.3114.8510.20.2 8.06 12.482. 6 0.07 0.10 0.05 100.661.06 2.619.210.1270.1340.008

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206Table C-2. Continued. Sample # Loc. Depth (m) SiO2 TiO2Al2O3FeOtMnOMgO*CaONa2OK2O P2O5**CR2O3TotalMg #Na8.0Fe8.0H2OS CL Weight percent by microprobe^ 2376-3/p1/ar2 B 2023 50.351.4815.329. 750.168.07 11.962.560.11 0.16 0.06 99.962.092.588.850.2100.1270.009 D1-3 B 3000 50.551.4315.2610.10.178.09 12.222.56 0.08 0.12 0.06 100.661.37 2.589.170.1670.1250.023 2378-2/p1 C 2223 50.541.3214.7310.20.198.10 12.442. 680.07 0.11 0.04 100.361.24 2.709.240.1300.1340.007 2378-8 C 2287 50.581.0815.349.050. 198.31 12.892.650.04 0.07 0.05 100. 264.512.738.480.0830.1180.002 2385-2 C ND 50.851.1215.129.320.2 8.31 12.762. 610.05 0.07 0.04 100.463.84 2.698.720.0920.1170.003 2376-5/ar1 B 3034 50.271.4115.579.470.188.34 12.232. 5 0.11 0.14 0.07 100.263.56 2.598.890.1830.1270.008 2376-5/ar2 B 3034 50.211.3615.619.630.228.36 12.022. 510.10 0.13 0.05 100.163.20 2.609.050.1820.1340.008 2376-7 B 3092 49.951.4115.9 9.550.178.54 11.772.53 0.10 0.13 0.06 100.163.91 2.679.180.1880.1290.016 2388-5 A-B 3797 50.581.1115.3 9.360.198.66 13.052. 270.05 0.07 0.06 100.664.67 2.439.130.0800.1190.002 2383-6 A 3661 50.421.1115.389.090.218.93 12.82.190 .06 0.07 0.06 100.366.05 2.429.180.0870.1240.000 2384-6 A-B 3707 49.250.9916.988.290.189.63 12.632. 4 0.03 0.05 0.05 100.469.70 2.699.210.0540.1000.001 D20-15/p1 A-B 3100 49.070.9317.468.040.199.80 12.36 2.390.02 0.06 0.05 100.370. 702.699.870.0640.1050.003 D20-13 A-B 3100 48.820.9817.238.170.149.81 12.432. 350.02 0.06 0.06 100.070.39 2.6510.000.0650.1100.002 D20-40 A-B 3100 49.140.9717.258.060.179.92 12.512. 370.02 0.06 0.04 100.570.90 2.689.900.0650.0990.002 D20-15/p3 A-B 3100 49.070.9417.438.110.159.96 12.42 .360.02 0.06 0.06 100.570. 852.679.960.0660.1020.002 2384-9 A-B 3623 49.371.0617.118.440.1910.0012.162. 440.03 0.07 0.05 100.970.11 2.7510.290.0750.1040.003 D20-15/p2 A-B 3100 48.990.9717.428.160.1310.0112.32 .370.03 0.06 0.05 100.470. 832.6810.010.0650.1120.002 2384-8 A-B 3648 49.261.0516.918.230.1510.0212.252. 410.03 0.06 0.06 100.470.68 2.7210.080.0800.1160.004 D20-1/p1 A-B 3100 49.290.9617.428.210.1310.0612.282. 4 0.03 0.06 0.06 100.870.80 2.7210.060.0640.1030.003 D20-7 A-B 3100 49.050.9817.298.030.1710.0712.52.3 90.02 0.06 0.06 100.671.28 2.719.880.0620.1080.002 D20-31 A-B 3100 49.140.9717.157.990.1510.1112.342. 370.02 0.06 0.04 100.371.47 2.699.850.0630.1050.003 2384-3 A-B 3751 49.080.9917.128.040.1710.1412.362. 330.03 0.07 0.06 100.371.41 2.659.900.0630.1070.003 D20-1/p2 A-B 3100 49.110.9817.298.070.1410.2312.262. 360.02 0.07 0.05 100.571.51 2.699.940.0660.1100.002 D20-20 A-B 3100 48.950.9517.1 8.160.1710.2412.182. 370.03 0.07 0.06 100.271.31 2.7010.030.0640.1050.001 Locations same as Table C-1. ^Major element analysis of Siqueiros natural glasses completed at the University of Tasmania usin g a Cameca SX50 electron microprobe (Danyushevsky, personal communication). *MgO contents have been corrected using MgO* = MgO 0.44722/0.90029. **P2O3 contents have been corrected using **P2O3 = P2O3 + 0.026281/0.82046. Na8.0 = Na2O contents normalized to 8.0 wt. % MgO. Fe8.0 = FeO contents normalized to 8.0 wt. % MgO.

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207Table C-3. ICP-MS major element analys is of Siqueiros tr ansform basalts. Sample Location Depth (m) SiO2TiO2 Al2O3FeO Fe2O3 MnO MgO CaO Na2OK2O P2O5 Total Na8.0 Fe8.0 ICP Major Element Analysis (wt. %) 2375-7 G B 2968 47.201.30 14.408.10 1. 00 0.16 7.79 11.302.56 0.08 0.10 95.00 2.52 7.88 2376-3 G B 3022 49.901.36 15.407.77 0. 96 0.16 8.33 11.002.52 0.13 0.13 98.60 2.57 8.10 2376-5 B 3034 49.701.34 16.708.02 0.99 0.16 8.59 11.602.49 0.14 0.11 100.80 2.58 8.59 2376-5 G B 3034 42.301.13 13.406.75 0. 83 0.14 7.35 9.80 2.14 0.10 0.11 84.90 2.02 6.03 2376-7 G B 3088 50.001.46 15.608.26 1. 02 0.17 8.24 11.302.61 0.12 0.12 99.90 2.65 8.50 2376-8 G B 3037 48.701.42 17.107.94 0. 98 0.15 7.03 11.402.54 0.11 0.13 98.50 2.35 6.82 2377-1 G B 3170 49.301.34 15.108.42 1. 04 0.17 8.64 12.102.57 0.10 0.10 99.90 2.67 9.04 2377-10 B 3085 49.601.89 16.609.56 1.18 0.19 7.46 11.002.63 0.17 0.18 101.60 2.53 8.96 2377-10 G B 3085 49.701.79 15.509.07 1. 12 0.18 7.27 11.402.69 0.14 0.17 100.20 2.55 8.25 2377-3 G B 3083 45.001.33 15.607.17 0. 88 0.14 6.23 10.802.34 0.09 0.12 90.60 1.96 4.97 2377-5 B 3090 49.802.03 15.709.31 1.15 0.19 7.37 10.902.86 0.18 0.19 100.80 2.74 8.62 2377-6 B 3011 49.601.89 16.008.91 1. 10 0.19 7.64 11.002.64 ND ND 100.40 2.58 8.52 2377-8 G B 3087 50.301.66 15.808.91 1.10 0.18 8.11 11.502.55 ND ND 101.50 2.57 9.02 2378-2 G C 2223 50.601.33 14.608.75 1. 08 0.18 7.81 12.002.66 0.08 0.09 100.30 2.63 8.55 2378-3 C 2234 50.201.35 14.908.99 1.11 0.18 8.11 12.402.66 0.09 0.08 101.20 2.68 9.10 2378-3 G C 2234 50.901.33 14.708.83 1. 09 0.18 8.01 12.202.67 0.08 0.09 101.20 2.67 8.84 2378-8 C 2287 51.001.10 15.307.94 0.98 0.17 8.70 12.802.67 0.07 0.08 101.80 2.77 8.60 2378-8 G C 2287 49.501.01 15.007.59 0. 94 0.15 8.30 12.202.60 ND 0.06 98.30 2.65 7.89 2380-11 G B 3079 49.601.87 15.709.23 1. 14 0.18 7.03 11.002.71 0.16 0.18 100.00 2.52 8.12 2380-12 G B 3069 49.201.94 14.409.56 1. 18 0.19 7.21 10.402.76 0.16 0.18 98.40 2.61 8.67 2380-9 G B 3135 49.801.55 17.308.26 1. 02 0.17 7.15 11.602.53 0.16 0.15 100.70 2.37 7.30 2381-11 G B-C 2686 51.501.27 14.408.58 1. 06 0.18 8.00 11.802.72 0.08 0.08 100.70 2.72 8.58 2383-6 G a A 3661 50.701.10 15.307.94 0.98 0.16 8.63 12.202.32 ND ND 100.50 2.41 8.54 2383-6 G b A 3661 50.901.09 15.207.94 0.98 0.16 8.79 12.202.31 ND ND 100.70 2.42 8.68 2384-3 G A-B 3751 48.800.70 14.207.06 0.87 0.13 17.10 9.07 1.98 ND 0.06 101.00 1.21 6.78 2384-6 G A-B 3707 47.900.78 14.006.92 0.85 0.13 14.30 10.002.00 ND 0.05 97.90 1.96 8.78 2384-9 G A-B 3623 48.000.89 14.207.31 0.90 0.13 15.10 9.60 2.03 ND 0.07 99.30 1.82 8.76 2385-2 G C 2352 51.201.15 14.808.18 1. 01 0.17 8.07 12.202.70 0.06 0.08 100.60 2.71 8.25 2385-3B G C 2333 49.701.29 14.608.99 1.11 0.20 7.52 11.702.70 ND ND 99.10 2.61 8.47 2386-5 G D 2178 51.801.13 14.908.18 1.01 0.18 8.34 12.602.54 ND ND 101.80 2.59 8.52

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208Table C-3. Continued. Sample Location Depth (m) SiO2TiO2 Al2O3FeO Fe2O3 MnO MgO CaO Na2O K2O P2O5Total Na8.0 Fe8.0 ICP Major Element Analysis (wt. %) 2388-10 G A-B 3057 51.401.11 15.008.10 1. 00 0.17 8.80 12.702.36 0.05 0.07 101.80 2.48 8.85 2389-4 A 3555 50.001.44 14.908.75 1. 08 0.18 8.02 12.102.39 ND ND 100.10 2.39 8.77 2389-5 A 3552 50.201.42 14.908.75 1. 08 0.18 8.10 11.402.32 ND ND 99.60 2.34 8.85 2389-5 G A 3552 50.101.32 14.308.75 1. 08 0.17 8.29 11.202.34 0.07 0.10 98.80 2.39 9.04 2390-3B WRTI 2934 49.902.14 15.908.50 1. 05 0.17 7.58 10.503.05 0.66 0.30 100.80 2.98 8.05 2390-5 G WRTI 3010 48.602.01 16.008.42 1.04 0.17 7.01 9.80 3.16 0.63 0.31 98.20 2.97 7.29 A25 D1-3 B 3000 50.101.36 15.308.50 1. 05 0.17 8.21 11.902.52 0.11 0.10 100.40 2.55 8.71 A25 D1-3 G B 3000 49.401.29 15.108. 34 1.03 0.18 7.95 11.702.48 ND ND 98.70 2.47 8.29 A25 D17-9 A-B 3000 50.501.71 15.409.15 1.13 0.18 7.87 11.402.57 0.13 0.14 101.30 2.55 9.01 A25 D18-2 G B 2800 49.201.74 14.109. 88 1.22 0.18 6.68 10.902.93 ND ND 98.30 2.66 8.31 *ICP-MS analysis was completed at the Geological Survey of Canada on Siqueiros. WR indicates whole rock samples. G indicates glass samples. Locations same as Table C-1. Mg# = Mg/(Mg + Fe2+); Fe2+ is assumed to be 0.9 Fe total. Na8.0 = Na2O contents normalized to 8.0 wt. % MgO. Fe8.0 = FeO contents normalized to 8.0 wt. % MgO.

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APPENDIX D TRACE ELEMENT CONTENTS OF THE SIQUEIROS SAMPLES

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210Table D-1. XRF trace element concentrations for the Siqueiros transform basalts. Sample # Dec. Lat. Dec. Long. Location*MgO (wt.%)CoCuGaNbNiRbSr YZnZrV Cr BaTi Sc K XRF analysis (ppm)^ 2375-7 8.34 -103.54 B 8.03 4580173.9821.4104 30928928039722820043.4 2375-9 8.34 -103.53 B 7.61 4779182.7881.6103 30928928244119810042.3 2376-10 8.35 -103.52 B 110 29 89 2376-3 8.37 -103.52 B 7.85 4676193.21073.1116 337410326538522820032.5 2376-5 8.36 -103.52 B 115 31 92 2376-6 8.35 -103.52 B 116 30 91 2376-7 8.35 -103.52 B 8.4 4477182.31112.1115 3413910128541422890037.1 2376-8 8.35 -103.52 B 8.02 4465193.71172.1134 378012527236425930033.5 2376-9 8.35 -103.52 B 117 29 84 2377-11 8.36 -103.52 B 7.14 4763215.4882.1120 52107164353360401280042.3 1300 2377-3 8.39 -103.52 B 7.72 4070183.5911.3114 368010828232615910033.9 2378-2 8.32 -103.31 C 8.43 4989182.4212.0100 327881 2378-6 8.35 -103.32 C 4883183.6262.3102 29778228241920790042.0 600 2379-2WR 8.39 -103.60 A-B 114 30 141 2380-11 8.36 -103.51 B 7.45 4865194.2843.5116 4692144342313391200036.7 1200 2380-12 8.36 -103.51 B 1.7114 46 147 2380-3 8.34 -103.50 B 1.1112 40 117 2380-4 8.34 -103.50 B 7.56 4568193.1712.3125 35112114 2380-5 8.35 -103.51 B 0.7126 35 113 2380-7 8.36 -103.51 B 1.0110 40 116 2380-9 8.36 -103.51 B 1.3114 41 126 2381-11 8.34 -103.44 B-C 7.76 4779183.2262.9101 29747527023112740044.0 600 2381-11WR 8.34 -103.44 B-C 7.76 27520416770041.4 1200 2381-3AWR 8.35 -103.42 B-C 3568193.4494.4112 24597222120810680029.2 1400 2382-10WR 8.30 -103.55 B 3966192.5850.9104 266780 2382-9 8.31 -103.55 B 0.9117 28 84 2383-2 8.36 -103.83 A 7.52 5073182.3331.7115 31849431322820880045.3 700 2383-6 8.37 -103.84 A 8.89 4581182.41111.789 26706625942311660039.8 400 2384-1 8.38 -103.66 A-B 9.6 1.3 63 18 45 2384-10 8.38 -103.67 A-B 9.59 0.381 25 61

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211Table D-1. Continued. Sample # Dec. Lat. Dec. Long. Location*MgO (wt.%)CoCuGaNbNiRbSr Y ZnZrV Cr BaTi Sc K XRF analysis (ppm) 2384-11 8.38 -103.68 A-B 8.79 5071172.32630.582 27 696724251617700032.7 300 2384-12 8.38 -103.68 A-B 9.11 0.8104 23 72 2384-13 8.38 -103.68 A-B 0.4114 35 97 2384-14 8.39 -103.68 A-B 2.997 34 111 2384-2 8.37 -103.66 A-B 9.54 0.8 0.471 21 49 2384-3 8.37 -103.66 A-B 10.12 1.2 0.673 22 51 2384-4 8.37 -103.66 A-B 1.4116 50 162 2384-6 8.37 -103.67 A-B 9.57 0.8 0.573 20 47 2384-7A 8.38 -103.67 A-B 9.9 1.0 71 21 48 2384-7B 8.38 -103.67 A-B 9.93 1.2 0.773 21 48 2384-8 8.38 -103.67 A-B 9.85 1.1 0.378 23 58 2384-9 8.38 -103.67 A-B 9.73 5373162.14310.175 23 63562028237 600026.8 200 2385-2 8.36 -103.32 C 8.21 4787182.8320.897 27 696925827416680083.6 400 2385-3A 8.36 -103.32 C 7.8 4274191.8611.2110 29 708427130921770042.6 500 2385-6T 8.36 -103.31 C 7.91 4779193.7241.7102 30 737927522713760046.8 500 2386-5 8.36 -103.13 D 8.23 3660171.7850.7108 27 597620828913680023.4 400 2387-1 8.35 -103.41 B-C (e) 4785173.6382.097 27 966826125512670043.0 500 2387-5 8.36 -103.39 B-C (e) 7.79 1.1102 31 87 2387-6 8.36 -103.38 B-C (e) 7.44 406 173.0431.998 28 707925730914760034.2 400 2388-1 8.35 -103.78 A-B 159 21 99 2388-10 8.37 -103.77 A-B 8.44 4691172.4723.279 25 696425343910640037.2 300 2388-14 8.38 -103.76 A-B 73 21 80 2388-7 8.37 -103.77 A-B 73 20 55 2389-1 8.39 -103.95 A 7.35 4460192.6831.186 36 818631832819920034.1 600 2390-1 8.30 -104.02 W-RTI 6.37 445519 16.213510.6287 34 811672762691441270027.5 5100 2390-3A 8.29 -104.03 W-RTI 9.0271 35 167 2390-3B 8.29 -104.03 W-RTI 9.1274 35 165 2390-4 8.30 -104.03 W-RTI 9.3273 35 166 2390-7 8.31 -104.05 W-RTI 1.6128 36 116 2390-8 8.32 -104.06 W-RTI 10.4277 33 169

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212Table D-1. Continued. Sample # Dec. Lat. Dec. Long. Location*MgO (wt.%)CoCuGaNbNiRbSr Y ZnZrV Cr BaTi Sc K XRF analysis (ppm) 2390-9 8.32 -104.06 W-RTI 5.54 535824 5.9 4.2121 59 120184509227761760054.4 1900 2391-1 8.35 -103.86 A-B 7.95 4770202.7860.989 33 808030530126850042.2 600 2391-5 8.34 -103.87 A-B 0.797 19 78 2391-9wr 8.33 -103.88 A-B 101 21 71 D1-3 8.34 -103.54 B 8.04 4580183.2870.7100 32 1108728039617820053.8 600 D1-5 8.34 -103.54 B 8.07 4476192.6810.699 32 748628238022810040.8 600 D18-1WR 8.32 -103.62 BW 4.7128 38 138 D18-2 8.32 -103.62 BW 2.1128 42 130 D18-3 8.32 -103.62 BW 6.72 4872204.5302.5125 40 9112833421 401120049.4 1200 D18-4 8.32 -103.62 BW 6.64 1.4126 42 132 D18-5 8.32 -103.62 BW 1.2127 42 130 D19-1 8.31 -103.64 BW 4774224.2241.3126 40 89126340182381110054.3 1200 D19-2 8.31 -103.64 BW 7.02 4870214.5311.1112 40 89120348194371120048.6 1000 D19-5wr 8.31 -103.64 BW 3.3 3.7109 32 104 D20-1 8.37 -103.66 A-B 10.01 6668131.07271.866 19 6047 D20-15 8.37 -103.66 A-B 9.89 7162130.8871 60 16 5943164132639460056.5 100 D22-1 8.37 -103.66 A-B 4771171.52771.788 24 64682295867 680031.2 200 D23-2 8.38 -103.67 A-B 9.42 4974172.12821.989 25 6869 D26-6 8.44 -103.36 C 7.04 4876203.0332.0116 35 8410328218629970045.7 1000 D27-5 8.44 -103.29 C-D 9.41 467617 2.52040.287 27 686723748514680032.1 300 D30-1 8.43 -103.91 E-RTI 7.35 4672203.3601.4113 39 86116347347251080047.4 700 D32-1 8.38 -103.29 C 7.69 4875192.3241.699 32 788929018718850042.5 500 D32-3 8.38 -103.29 C 7.34 4774202.9471.3110 34 859629729825910019.0 700 D33-1 8.39 -103.26 C 4877193.5341.098 32 8287 D34-2 8.39 -103.17 C-D 9.12 457514 3.01731.2111 21 606221045111540028.0 600 D35-3 8.38 -103.81 A 7.77 4767193.3931.1113 41 84127314336231060041.7 900 D35-4 8.38 -103.81 A 7.88 4766182.71140.9120 41 80126302396291040038.2 800 D36-3 8.41 -103.76 A 8.39 4575192.6950.389 30 727826439323730039.6 600 D38-1 8.37 -103.92 A 8.78 4785192.0910.685 28 727225740916690037.1 400 D38-2 8.37 -103.92 A 7.66 4666203.1831.3102 38 87108339326281030052.5 800

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213Table D-1. Continued. Sample # Dec. Lat. Dec. Long. LocationMgO (wt.%)CoCuGaNbNiRbSr Y ZnZrV Cr BaTi Sc K XRF analysis (ppm) D4-3 8.38 -103.51 B 0.6104 36 99 D44-1 8.38 -103.11 D 7.99 4784182.2342.1101 25 407425842816670038.7 500 D4-6 8.38 -103.51 B 7.49 4673182.3432.4106 35 8410032328726950050.0 800 D5-5 8.39 -103.46 B 1.6123 37 111 D6-1 8.40 -103.44 BE 8.03 5282242.51321.2106 30 789127035136820039.6 600 D8-1wr 8.33 -103.60 BW 2.4122 34 118 Locations A = spreading center A, A-B = fault separating spread ing centers A and B, B = spread ing center B, B-C = fault separ ating spreading centers B and C, C = spreading center C, C-D = fault separating spreading center C and trough D, D = trough D, WRTI = western ridge transform intersection, ERTI = eastern ridge transform intersection. XRF whole rock and glass powders analyzed at the University of Florida. MgO values from Table C-1.

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214Table D-2. ICP Trace element concentrations for the Siqueiros transform basalts. Sample # Dec. Lat. Dec. Long. Location*MgO wt. %LaCe Nd SmEu Gd Dy Y Er Yb Lu Sc (Ce/Yb)n ICP Analysis (ppm) 2375-7 8.34 -103.54 BW 8.03 2.538.618. 17 2.791.05 3.64 30.8 2.77 36.20.86 2375-9 8.34 -103.53 BW 7.61 2.678.848. 77 2.891.12 4.33 33.2 3.16 40.30.78 2376-3 8.37 -103.52 BS 7.85 3.4410.819. 98 3.251.19 4.55 34.0 3.17 36.00.95 2376-7 8.35 -103.52 BE 8.4 3.3610.679. 4 3.2 1.19 4.7 34.6 3.27 37.30.91 2376-8 8.35 -103.52 BS 8.02 3.711.9410. 953.631.28 5.1 38.0 3.56 34.80.93 2377-11 8.36 -103.52 BN 7.14 5.7317.5415. 865.241.68 7.32 54.2 5.08 39.00.96 2377-3 8.39 -103.52 BN 7.72 3.4211.0510. 053.481.23 4.91 37.1 3.54 36.40.87 2378-2 8.32 -103.31 CW 8.43 2.197.637. 21 2.340.98 3.94 31.0 2.94 41.20.72 2378-6 8.35 -103.32 CW 2.347.867. 88 2.711.08 4.14 31.4 2.99 42.00.73 2378-7 8.35 -103.32 CW 7.76 2.658.457. 91 2.581.14 4.04 31.8 3.01 40.20.78 2379-2WR 8.39 -103.60 A-B 3.1110.26 9.18 3.261.39 4.95 38.7 3.94 41.70.72 2380-11 8.36 -103.51 BE 7.45 4.7714.8213. 584.4 1.47 6.34 46.8 4.52 37.80.91 2380-4 8.34 -103.50 BE 7.56 3.1110.668. 9 2.931.1 4.43 34.1 3.2 36.50.93 2380-7 8.36 -103.51 BE 3.5712.3410.993.66 1.39 5.1 6.3439.94.223.8 0.5944.00.90 2381-11 8.34 -103.44 B-C 7.76 1.947.02 7.21 2.051 3.67 29.2 2.77 41.80.70 2381-3AWR 8.35 -103.42 B-C 1.415.87 5.75 2.030.89 3.15 25.1 2.4 35.20.68 2382-10WR 8.30 -103.55 B (SW) 0.843.34 3.23 1.060.45 1.64 13.3 1.27 18.00.73 2382-7 8.32 -103.55 B (SW) 7.39 3.5311.39 10.7 3.621.32 5.09 38.2 3.61 42.00.88 2383-2 8.36 -103.83 A 7.52 2.318.537. 61 2.321.05 3.95 32.0 2.98 38.90.80 2383-6 8.37 -103.84 A 8.89 1.696.12 6 2.180.92 3.46 27.5 2.63 39.00.65 2384-1 8.38 -103.66 A-B 9.6 0.813.364. 14 1.810.64 2.51 18.9 1.76 25.30.53 2384-11 8.38 -103.68 A-B 8.79 1.565.976. 55 2.390.95 3.64 27.9 2.61 34.40.64 2384-3 8.37 -103.66 A-B 10.12 1.114.865.26 2. 080.81 2.833.5422.22.392.070.3329.00.65 2384-9 8.38 -103.67 A-B 9.73 0.984.535. 04 1.950.68 2.95 22.7 2.13 29.00.59 2385-2 8.36 -103.32 C 8.21 1.726.316. 77 2.320.95 3.57 28.1 2.62 42.30.67 2385-6T 8.36 -103.31 C 7.91 1.976.976. 97 2.461.04 3.79 30.0 2.82 41.50.69 2386-5 8.36 -103.13 D 8.23 1.326.334. 85 1.580.51 2.63 23.4 2.28 37.80.77 2387-1 8.35 -103.41 B-C (e) 1.756.62 6.94 2.230.95 3.46 26.7 2.45 30.30.75 2387-6 8.36 -103.38 B-C (e) 7.44 1.86.897 .42 2.571.09 3.97 30.7 2.89 37.40.66 2388-1 8.35 -103.78 A-B 1.325.314.65 1. 680.89 2.613.4921.92.4 2.320.3742.70.64

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215Table D-2. Continued. Sample # Dec. Lat. Dec. Long. Location*MgO wt. %La Ce Nd SmEu Gd Dy Y Er Yb Lu Sc (Ce/Yb)n ICP Analysis (ppm) 2388-14 8.38 -103.76 A-B 1.164.593.99 1. 510.75 2.453.4322.02.4 2.280.3440.20.56 2388-7 8.37 -103.77 A-B 0.783.7 3.69 1. 590.8 2.443.2520.52.232.050.3138.70.50 2389-1 8.39 -103.95 A 7.35 2.358.268. 33 3.031.18 4.88 37.6 3.6 37.50.64 2390-1 8.30 -104.02 W-RTI 6.37 11.5827.37 14.853.771.31 4.79 31.1 2.78 25.62.73 2390-3A 8.29 -104.03 W-RTI 13.2532.0919.9 4. 861.69 5.555.9435.93.663.260.4930.62.73 2390-4 8.30 -104.03 W-RTI 13.7232.2120.425. 081.71 5.646.1136.63.753.310.5 31.32.70 2390-5 8.31 -104.04 W-RTI 14.1733.1520.065. 031.75 5.545.8335.03.6 3.120.4729.12.95 2390-7 8.31 -104.05 W-RTI 3.9113.0511.413. 691.38 4.865.8836.63.973.450.5340.31.05 2390-8 8.32 -104.06 W-RTI 14.3433.4720.475. 3 1.76 5.736.0536.33.793.280.4930.32.83 2390-9 8.32 -104.06 W-RTI 5.54 7.1921.46 18.625.962.05 8.18 59.8 5.73 42.21.04 2391-10wr 8.32 -103.88 A-B 0.793.432.88 1. 160.65 1.9 2.5716.31.811.690.2640.20.56 2391-5 8.34 -103.87 A-B 0.863.663.17 1. 210.65 2.073.0219.22.142.070.3140.60.49 2391-9wr 8.33 -103.88 A-B 1.295.234.91 1. 730.81 2.693.5522.02.382.140.3244.30.68 D1-5 8.34 -103.54 B 8.07 2.518.598. 4 2.661.1 4.14 32.2 3.06 39.20.78 D17-1WR 8.40 -103.60 A-B 2.8 9.4 9. 19 2.881.16 4.54 34.5 3.32 38.30.79 D18-1WR 8.32 -103.62 BW 3.2210.6110. 283.251.34 5.09 38.0 3.61 41.50.82 D18-3 8.32 -103.62 BW 6.72 4.5914.0112. 633.831.5 5.62 41.6 3.96 42.80.98 D19-1 8.31 -103.64 BW 4.2113.2811. 543.391.24 5.45 39.9 3.73 41.50.99 D19-2 8.31 -103.64 BW 7.02 3.4511.5910. 133.431.3 5.11 39.1 3.63 40.30.89 D20-1 8.37 -103.66 A-B 10.01 0.773.674. 3 1.670.69 2.56 19.5 1.8 25.40.57 D20-15 8.37 -103.66 A-B 9.89 0.773.474. 04 1.720.67 2.55 18.5 1.74 25.10.55 D20-5 8.37 -103.66 A-B 1.2 5.2 5 2 0.86 2.9 3.5 23.02.3 2.1 0.3230.00.69 D22-1 8.37 -103.66 A-B 1.4 5.516. 37 2.110.93 3.42 26.4 2.46 32.30.62 D22-3 8.37 -103.66 A-B 9.47 1.385.516. 47 2.250.89 3.44 26.5 2.45 32.60.62 D23-2 8.38 -103.67 A-B 9.42 1.435.556. 56 2.370.96 3.57 26.8 2.53 33.20.61 D25-6 8.39 -103.41 CW 7.37 3.2610.910. 253.061.29 4.76 36.0 3.4 42.20.89 D27-5 8.44 -103.29 C-D 9.41 1.9 6.3 6. 63 2.281.03 3.67 27.5 2.7 34.90.65 D30-1 8.43 -103.91 E-RTI 7.35 3.5211.41 10.313.291.43 5.16 40.1 3.87 41.90.82 D32-1 8.38 -103.29 CE 7.69 2.548.137. 75 2.751.19 4.26 32.5 3.21 42.40.70 D32-3 8.38 -103.29 CE 7.34 2.879.439. 32 2.781.24 4.55 34.5 3.4 41.20.77

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216Table D-2. Continued. Sample # Dec. Lat. Dec. Long. Location*MgO wt. %LaCe Nd Sm Eu GdDy Y Er Yb LuSc (Ce/Yb)n ICP Analysis (ppm) D33-1 8.39 -103.26 CE 2.288.05 7. 94 2.781.12 4.23 32.5 3.05 41.60.73 D34-2 8.39 -103.17 C-D 9.12 2.216.78 5. 88 1.750.82 2.94 22.4 2.2 34.50.86 D35-4 8.38 -103.81 AE 7.88 3.7712.5211. 633.711.38 5.43 40.2 3.81 38.60.91 D36-3 8.41 -103.76 AE 8.39 2.297.63 7. 36 2.561.01 3.93 30.8 2.92 38.40.73 D38-2 8.37 -103.92 A 7.66 3.2810.6710. 443.291.26 5.08 39.4 3.71 40.30.80 D44-1 8.38 -103.11 DW 7.99 2.037.13 6. 61 2.130.91 3.37 26.7 2.53 40.60.78 D4-6 8.38 -103.51 B 7.49 3.0810.079. 96 3.281.28 5.03 37.5 3.55 42.90.79 D6-1 8.40 -103.44 BE 8.03 2.899.02 7. 35 2.270.94 3.9 29.1 2.77 35.60.90 D8-1wr 8.33 -103.60 BW 2.478.62 8. 72 2.851.27 4.56 14.8 3.42 44.40.70 *Locations same as Table D-1. ICP-MS analysis completed at the Univer sity of Houston by Dr. Jack Casey. MgO values from Table C-1.

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217Table D-3. DCP trace element concentrations for the Siqueiros transform basalts. Sample # Dec. Lat. Dec. Long Location*MgO wt.%TiO2 wt.%K2O wt%Mn Ba Cr CuNi Sc Sr V Y Zr DCP Analysis (ppm) 2384-1 8.38 -103.66 A-B 12.78 0.93 0.04 0.152 165980 33831 73 1932351 2384-3 8.37 -103.66 A-B 10.49 1.00 0.03 0.152 498 82 21632 79 2072556 2384-7B 8.38 -103.67 A-B 10.63 0.90 0.04 0.151 647 88 21530 76 2032351 2384-8 8.38 -103.67 A-B 10.34 1.04 0.04 0.151 635 78 20332 81 2172560 2390-5 8.31 -104.04 W-RTI 7.74 2. 02 0.64 0.16141 219 55 12329 31826334177 D20-13 8.37 -103.66 A-B 10.60 0.95 0.03 0.142 640 83 21432 77 2252552 D20-15 8.37 -103.66 A-B 11.24 1.00 0.04 0.1512 601 90 22933 82 2162758 D20-5 8.37 -103.66 A-B 10.63 0.93 0.03 0.143 534 85 21831 75 2163 54 RC-41 E-RTI 8.00 1.46 0.08 0.185 303 79 88 41 11430033101 *Locations same as Table D-1. DCP analysis completed on phenoc rysts-free samples at Lamont Doherty Earth Observatory. MgO values from Table C-1.

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218 Table D-4. ICP trace element concentrations of the Siqueiros transform basalts. Sample 2375-7G 2376-3G 2376-52376-5G2376-7G2376-8G2377-1G 2377-10 2377-10G Location* B B B B B B B B B ICP Trace Element Analysis (ppm) Ag 0.10 0.10 ND ND ND 0.20 0.30 0.10 ND Ba ND ND ND ND ND ND ND ND ND Be ND ND ND ND ND ND ND ND ND Bi ND ND ND ND ND ND 0.2 ND ND Cd 1.1 ND ND 0.2 0.3 0.5 0.5 0.2 0.2 Cl 207 ND 700 1036 117 2008 887 826 ND Co 38 36 37 37 38 33 38 35 35 Cr 334 331 348 349 327 289 341 266 269 Cs ND ND ND ND ND ND ND ND ND Cu 77 72 76 75 73 60 79 61 61 F 123 ND 126 127 136 137 113 182 ND Ga 16 16 16 16 16 16 15 19 17 Hf 2.30 2.60 2.40 2.40 2. 60 3.10 2.10 3.50 3.30 In 0.09 ND 0.08 ND ND 0.07 0.36 0.08 ND Mo ND 0.2 ND 0.2 0.3 0.4 0.4 0.3 0.3 Nb 2.00 2.60 2.30 2.40 2. 60 2.60 2.50 3.50 3.50 Ni 85 106 108 113 101 105 92 91 92 Rb 0.59 0.77 0.83 0.60 0. 81 1.10 1.10 1.20 1.10 Sb ND ND ND ND ND ND ND ND ND Sc 37 33 33 33 33 30 33 35 35 Sn 0.80 1.00 1.30 0.70 0. 90 1.20 1.10 1.10 1.00 Sr 95 109 108 109 106 115 90 109 111 Ta 0.13 0.20 0.16 0.18 0. 21 0.19 0.14 0.25 0.24 Te ND ND ND ND ND ND ND ND ND Th 0.12 0.16 0.14 0.14 0. 16 0.15 0.12 0.21 0.2 Tl ND ND ND ND ND ND ND ND ND U 0.05 0.07 0.06 0.06 0. 07 0.08 0.06 0.09 0.09 V 281 259 257 258 273 255 259 319 318 Zn 71 70 68 67 71 72 67 85 84 Zr 84 97 93 90 100 121 77 134 130 Ce 8.50 10.38 9.43 9.36 10. 20 10.98 7.83 14.09 13.78 Dy 5.09 5.22 4.98 4.89 5. 43 5.49 4.69 7.04 6.92 Er 3.03 3.05 2.87 2.79 3. 13 3.20 2.73 4.08 4.12 Eu 1.10 1.17 1.12 1.08 1. 19 1.20 1.04 1.45 1.47 Gd 4.44 4.57 4.27 4.26 4. 74 4.86 4.08 6.23 6.16 Ho 1.11 1.11 1.05 1.04 1. 16 1.18 1.00 1.49 1.50 La 2.61 3.34 3.02 2.97 3. 21 3.43 2.64 4.40 4.40 Lu 0.48 0.48 0.45 0.46 0. 50 0.52 0.44 0.68 0.67 Nd 8.35 9.65 8.93 8.60 9. 44 9.84 7.63 12.56 12.45 Pr 1.52 1.78 1.60 1.59 1. 72 1.86 1.37 2.35 2.31 Sm 3.07 3.30 3.17 2.98 3. 35 3.41 2.86 4.34 4.48 Tb 0.82 0.83 0.78 0.77 0. 86 0.87 0.73 1.12 1.12 Tm 0.48 0.49 0.46 0.46 0. 50 0.52 0.44 0.66 0.68 Y 31.1 31.8 29.3 29.5 31. 9 33.5 27.5 42.4 42.1 Yb 3.25 3.21 2.95 2.94 3. 30 3.29 2.84 4.30 4.41 (Ce/Yb)n 0.73 0.90 0.89 0.89 0. 86 0.93 0.77 0.91 0.87

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219 Table D-4. Continued. Sample 2377-3G 2377-5 2377-62377-8G2378-2G2378-32378-3G2378-8 2378-8G 2380-11G Location* B B B B C C C C C B ICP Trace Element Analysis (ppm) Ag 0.10 0.20 0.30 ND 0.10 ND ND ND ND ND Ba ND ND ND ND ND ND ND ND ND ND Be ND 0.5 ND ND ND ND ND ND ND 0.50 Bi ND ND ND ND ND ND ND ND ND ND Cd 0.3 0.6 ND ND 0.20 ND ND 0.20 0.30 1.20 Cl 1453 ND 557 ND ND ND ND 165 223 ND Co 31 33 35 36 39 39 39 37 37 35 Cr 283 259 285 309 319 328 333 393 399 254 Cs ND ND ND ND ND ND ND ND ND ND Cu 62 53 57 65 83 83 86 93 93 59 F 137 ND 176 ND 112 114 115 96 93 ND Ga 16 19 19 17 16 15 16 14 15 17 Hf 2.60 4.10 3.40 2.80 2.10 2.10 2.20 1.70 1.80 3.60 In ND 0.09 ND ND ND ND ND ND ND 0.13 Mo ND 0.4 ND ND ND ND ND ND ND 0.70 Nb 2.40 3.80 3.50 3.00 1.90 2.00 2.00 0.95 0.96 3.80 Ni 81 95 92 106 42 46 47 73 73 89 Rb 0.89 1.60 1.40 1.10 0.79 0.87 0.74 0.34 0.49 1.50 Sb ND ND ND ND ND ND ND ND ND ND Sc 31 34 34 35 39 38 39 39 39 35 Sn 2.80 1.60 1.50 1.20 1.00 1.00 0.80 ND 1.00 1.90 Sr 108 111 107 109 92 91 93 92 91 112 Ta 0.17 0.25 ND ND 0.13 0. 16 0.14 0.19 0.11 0.26 Te ND ND ND ND ND ND ND ND ND ND Th 0.15 0.24 ND ND 0.11 0. 11 0.11 0.06 0.06 0.22 Tl ND ND ND ND ND ND ND ND ND ND U 0.06 0.11 ND ND 0.05 0. 05 0.05 0.03 0.03 0.10 V 264 319 320 303 282 276 281 237 236 321 Zn 68 90 83 78 72 70 72 70 59 86 Zr 99 154 132 110 76 76 81 62 68 138 Ce 10.14 16.07 13.8312.41 7.86 7.88 7.80 5.54 5.58 14.61 Dy 5.53 7.89 6.67 6.20 4.87 4.83 4.90 4.04 3.96 7.13 Er 3.18 4.60 3.91 3.59 2.85 2.80 2.88 2.43 2.35 4.24 Eu 1.18 1.62 1.38 1.32 1.08 1.07 1.08 0.94 0.92 1.50 Gd 4.85 7.02 5.97 5.43 4.24 4.14 4.14 3.47 3.41 6.27 Ho 1.19 1.70 1.44 1.32 1.08 1.03 1.05 0.87 0.86 1.55 La 3.16 4.92 4.29 3.94 2.39 2.37 2.36 1.54 1.55 4.62 Lu 0.51 0.74 0.62 0.58 0.47 0.44 0.45 0.38 0.38 0.67 Nd 9.55 14.46 12.3710.72 7.65 7.83 7.94 6.30 6.18 13.09 Pr 1.78 2.66 2.31 2.07 1.39 1.41 1.42 1.08 1.07 2.40 Sm 3.38 5.10 4.32 3.84 2.88 2.82 2.79 2.37 2.37 4.59 Tb 0.87 1.26 1.06 0.96 0.78 0.76 0.76 0.64 0.65 1.16 Tm 0.52 0.74 0.62 0.60 0.46 0.46 0.46 0.39 0.38 0.68 Y 32.9 47.3 41.0 37.3 29.8 29.6 30.0 25.1 24.8 44.3 Yb 3.39 4.88 4.13 3.73 3.02 2.98 2.97 2.49 2.39 4.49 (Ce/Yb)n 0.83 0.91 0.93 0.92 0.72 0.74 0.73 0.62 0.65 0.90

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220 Table D-4. Continued. Sample 2380-12G 2380-9G2381-11G2383-6G a2383-6G b2384-3G 2384-6G 2384-9G Location* B B B-C A A A-B A-B A-B ICP Trace Element Analysis (ppm) Ag ND ND 0.30 ND ND ND 0.30 ND Ba ND ND ND ND ND ND ND ND Be 0.50 ND ND ND ND ND ND ND Bi ND ND ND ND ND ND ND ND Cd ND ND 0.2 ND 1 ND ND ND Cl ND ND ND ND ND ND ND 1187 Co 36 37 39 38 39 60 51 51 Cr 277 273 189 375 377 594 739 924 Cs ND ND ND ND ND ND ND ND Cu 61 64 79 80 79 62 80 69 F 197 122 106 132 85 62 67 73 Ga 18 17 16 15 15 11 12 12 Hf 3.70 3.00 1.90 1.70 2.20 1.30 1.30 1.40 In 0.06 0.1 ND ND ND ND ND ND Mo 0.40 0.3 0.2 ND 1 0.7 ND ND Nb 3.90 3.10 1.80 1.10 1.10 0.48 ND 0.52 Ni 92 94 44 106 110 753 423 547 Rb 1.40 1.20 0.85 0.52 0.57 0.30 ND 0.26 Sb ND ND ND ND ND 0.2 ND ND Sc 36 33 39 36 35 23 24 25 Sn 1.30 2.80 0.70 0.80 0.50 ND 0.90 ND Sr 111 114 94 81 81 58 61 66 Ta 0.23 0.20 0.13 ND ND ND ND 0.06 Te ND ND ND ND ND ND ND ND Th 0.22 0.2 0.1 ND ND ND ND ND Tl ND ND ND ND ND ND ND ND U 0.10 0.08 ND ND ND ND ND ND V 335 293 276 264 263 159 175 189 Zn 90 79 72 67 67 56 74 61 Zr 144 120 74 62 85 45 44 47 Ce 15.41 12.25 7.02 5.91 5.75 3.39 3.51 4.11 Dy 7.49 6.09 4.57 4.22 4.19 2.97 3.00 3.39 Er 4.47 3.64 2.69 2.53 2.46 1.72 1.78 1.97 Eu 1.57 1.30 1.03 0.91 0.89 0.66 0.69 0.75 Gd 6.54 5.31 3.91 3.46 3.46 2.51 2.58 2.93 Ho 1.64 1.32 0.99 0.90 0.87 0.64 0.65 0.72 La 4.80 3.82 2.11 1.78 1.72 0.94 0.90 1.10 Lu 0.70 0.58 0.43 0.39 0.39 0.28 0.28 0.32 Nd 14.00 10.85 7.01 6.19 6.08 4.19 4.53 4.98 Pr 2.59 2.04 1.26 1.07 1.02 0.68 0.73 0.83 Sm 4.79 3.90 2.65 2.38 2.28 1.68 1.75 1.95 Tb 1.21 1.00 0.71 0.66 0.65 0.47 0.48 0.54 Tm 0.72 0.59 0.42 0.40 0.39 0.27 0.28 0.31 Y 46.7 38.0 28.2 26.3 25.5 18.3 18.4 20.9 Y b 4.63 3.87 2.772.552.531.771.71 1.98(Ce/Yb)n 0.92 0.88 0.70 0.64 0.63 0.53 0.57 0.58

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221 Table D-4. Continued. Sample 2385-2G 2385-3B G 2386-5G2387-52388-10G2389-42389-52389-G 2390-3B2390-5G Location* C C D B-CA-B A A A WRTIWRTIICP Trace Element Analysis (ppm) Ag 0.10 ND ND ND ND ND ND ND ND 0.20 Ba ND ND ND ND ND ND ND ND 125 129 Be ND ND ND ND ND ND ND ND 0.90 0.90 Bi ND ND ND ND ND ND ND ND ND ND Cd ND ND ND 2 ND ND ND 1.10 0.20 0.30 Cl ND ND ND 247ND 2181333417 ND 271 Co 38 40 40 37 37 36 37 38 32 33 Cr 231 146 231 297408 251243271 214 205 Cs ND ND ND ND ND ND ND ND 0.11 0.13 Cu 88 77 87 70 93 66 69 66 51 50 F 93 115 98 11178 110106105 313 330 Ga 15 16 15 16 15 17 17 17 18 18 Hf 1.70 2.10 1.80 2.301. 70 2.202.502.10 4.00 4.10 In ND ND ND ND 0.07 ND ND 0.05 0.06 0.15 Mo ND ND ND ND ND ND ND 0.30 1.00 1.50 Nb 1.20 1.90 1.60 1.600. 98 1.501.601.60 19.0020.00 Ni 49 43 58 74 78 83 86 84 107 107 Rb 0.62 0.73 0.83 0.960. 34 ND 0.820.59 11.0012.00 Sb ND ND ND ND ND ND ND ND ND ND Sc 39 41 40 34 36 37 36 36 28 27 Sn 0.70 0.60 0.50 0.800. 70 0.801.000.80 1.40 1.90 Sr 95 103 93 91 70 74 70 70 269 295 Ta 0.09 ND ND ND 0.07 ND ND 0.11 0.99 1.10 Te ND ND ND ND ND ND ND ND ND ND Th ND ND ND ND 0.06 ND ND 0.09 1.20 1.30 Tl ND ND ND ND ND ND ND ND 0.02 0.02 U ND ND ND ND 0.03 ND ND 0.04 0.38 0.42 V 261 293 263 255253 312308310 260 255 Zn 65 76 67 69 65 77 77 77 79 78 Zr 62 77 67 84 57 77 89 76 167 174 Ce 6.35 8.13 6.75 7.725. 33 8.017.897.85 34.0836.18 Dy 4.18 5.02 4.21 4.924. 23 5.455.405.26 6.03 5.81 Er 2.53 2.94 2.51 2.892. 48 3.233.243.15 3.26 3.18 Eu 0.95 1.12 0.91 1.100. 87 1.111.081.06 1.72 1.73 Gd 3.68 4.30 3.59 4.283. 56 4.494.384.36 6.01 5.88 Ho 0.91 1.08 0.94 1.070. 92 1.161.181.16 1.22 1.20 La 1.82 2.40 2.03 2.181.55 2.322.312.35 13.7114.82 Lu 0.40 0.48 0.39 0.440.39 0.530.530.52 0.51 0.50 Nd 6.75 8.19 6.68 8.226. 14 8.388.238.24 20.7521.70 Pr 1.19 1.48 1.20 1.471. 03 1.461.441.43 4.66 4.85 Sm 2.53 2.98 2.39 3.012. 28 2.972.942.91 5.18 5.15 Tb 0.66 0.79 0.66 0.800. 64 0.850.820.83 0.95 0.96 Tm 0.40 0.47 0.40 0.460. 40 0.540.520.52 0.51 0.50 Y 26.4 31.1 26.5 30.526.7 34.434.434.5 36.0 35.2 Yb 2.60 3.00 2.55 2.932. 59 3.403.413.36 3.23 3.17 (Ce/Yb)n 0.68 0.75 0.74 0.73 0.57 0.660.640.65 2.93 3.17

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222 Table D-4. Continued. Sample A25 D1-3 G A25 D17-9A25 D18-2 G Location* B A-B B ICP Trace Element Analysis (ppm) Ag 0.40 0.10 0.20 Ba ND ND ND Be ND ND 1 Bi ND ND ND Cd 5 0.20 ND Cl 1393 106 218 Co 38 37 38 Cr 332 285 124 Cs ND ND ND Cu 76 66 70 F 123 157 174 Ga 16 17 18 Hf 2.20 2.90 3.10 In ND 0.06 ND Mo ND 1.00 ND Nb 2.00 2.70 3.80 Ni 84 105 40 Rb 0.61 0.83 1.40 Sb ND ND ND Sc 36 34 39 Sn 1.00 1.00 1.00 Sr 94 92 120 Ta ND 0.19 ND Te ND ND ND Th ND 0.17 ND Tl ND ND ND U ND 0.07 ND V 278 311 327 Zn 71 109 87 Zr 81 106 121 Ce 8.36 11.35 13.68 Dy 5.07 6.24 6.50 Er 2.98 3.60 3.81 Eu 1.10 1.33 1.49 Gd 4.29 5.48 5.83 Ho 1.10 1.35 1.39 La 2.56 3.46 4.34 Lu 0.48 0.58 0.61 Nd 8.19 10.71 11.81 Pr 1.46 1.95 2.28 Sm 2.95 3.90 4.16 Tb 0.79 1.00 1.04 Tm 0.47 0.58 0.61 Y 31.0 37.9 38.9 Yb 3.11 3.82 3.97 (Ce/Yb)n 0.75 0.82 0.96 *Locations same as Table D-1. ICP-MS completed at the Geological Survey of Canada. MgO values from Table C-1.

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APPENDIX E FRACTIONAL CRYSTALLIZATION MODE L PARAMETERS CALCULATED IN PETROLOG

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224Table E-1. 2377-7P at low pressure. Step Melt% T(C) SiO2 TiO2 Al2O3 FeO MgO CaO Na2OK2O P2O5 Fo An Olv% Plg% Cpx% 1 100.00 1258 48.50 1.19 17.417.77 10.5011.16 2.30 0.09 0.13 88.90-1 0.00 0.00 0.00 2 99.00 1249 48.58 1.20 17.597.76 10.1211.27 2.32 0.09 0.13 88.53-1 1.00 0.00 0.00 5 96.99 1235 48.71 1.23 17.797.76 9.58 11.42 2.36 0.09 0.13 87.9283.462.53 0.49 0.00 7 94.98 1233 48.78 1.25 17.647.85 9.51 11.40 2.38 0.10 0.14 87.7082.873.07 1.95 0.00 12 89.96 1229 48.96 1.32 17.278.08 9.31 11.37 2.44 0.10 0.15 87.0981.364.45 5.59 0.00 17 84.93 1224 49.16 1.40 16.888.33 9.08 11.34 2.49 0.11 0.15 86.4079.755.84 9.23 0.00 22 79.91 1218 49.36 1.49 16.448.60 8.84 11.33 2.54 0.11 0.16 85.6278.027.23 12.850.00 27 74.90 1212 49.57 1.59 15.978.88 8.57 11.34 2.59 0.12 0.17 84.7476.178.64 16.460.00 32 69.87 1204 49.79 1.70 15.469.18 8.26 11.37 2.63 0.13 0.19 83.7174.1710.0620.070.00 37 64.85 1195 50.02 1.84 14.899.49 7.91 11.44 2.67 0.14 0.20 82.5172.0011.5023.650.00 42 59.83 1183 50.27 1.99 14.279.81 7.51 11.55 2.70 0.15 0.22 81.0869.6512.9727.200.00 47 54.82 1168 50.55 2.17 13.5910.137.02 11.72 2.73 0.16 0.24 79.3167.1014.4930.690.00 50 52.81 1162 50.64 2.25 13.3510.276.82 11.74 2.74 0.17 0.25 78.5366.1114.9731.950.27 53 49.81 1158 50.62 2.37 13.1910.576.63 11.50 2.79 0.18 0.26 77.4565.0115.1133.341.75 58 44.80 1150 50.58 2.59 12.8811.116.27 11.05 2.89 0.20 0.29 75.3462.9915.3435.634.23 63 39.79 1140 50.51 2.86 12.5711.675.84 10.54 3.00 0.23 0.33 72.7560.7515.5637.886.77 68 34.78 1127 50.40 3.20 12.2312.235.32 9.93 3.13 0.26 0.37 69.4958.2315.7740.099.36 71 32.22 1119 50.32 3.40 12.0612.485.00 9.57 3.20 0.28 0.40 67.4956.8215.8841.1910.71 Notes: The relative proportions (in wt. %) of the major element oxides an d crystallizing phases are s hown at ~5 wt. % increment s of crystallization, except when a new mineral comes on the liquidus. Olv = olivine; Plg = plagiocl ase; Cpx – clinopyroxene; Spl = spinel; Fo = forsterite content of crystal lizing olivine; An = anorthite content of crystallizing plagioclase. The oxides and parameters not used in the liquid line of descent modeling have been omitted.

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225Table E-2. D34-2P at low pressure. Step Melt% T(C) SiO2 TiO2 Al2O3FeOMgOCaONa2OK2OP2O5 Cr2O3 Fo An Olv%Plg%Cpx%Spl% 1 100.00 1297 49.59 1.02 16.197.64 9.0712.892.170.060.05 0.131 -1 -1 0.000.00 0.00 0.00 2 99.76 1214 49.72 1.02 16.177.82 9.0612.922.170.060.05 0.040 87.05-1 0.000.00 0.00 0.24 3 99.46 1211 49.76 1.02 16.227.81 8.9512.962.180.060.05 0.037 86.9180.840.290.00 0.00 0.25 4 99.00 1211 49.77 1.02 16.187.83 8.9312.972.180.060.05 0.037 86.8580.710.420.34 0.00 0.25 8 94.98 1206 49.94 1.07 15.867.99 8.7413.032.210.060.05 0.038 86.3279.501.553.22 0.00 0.25 13 89.96 1200 50.17 1.13 15.438.21 8.4913.122.240.070.06 0.039 85.5877.912.986.81 0.00 0.25 22 81.92 1187 50.54 1.24 14.708.57 8.0513.292.280.070.06 0.040 84.1975.205.1912.440.21 0.25 24 79.91 1186 50.56 1.26 14.638.70 7.9913.202.300.080.06 0.035 83.8674.745.3013.391.14 0.25 29 74.90 1183 50.61 1.33 14.449.04 7.8212.962.360.080.07 0.025 82.9673.555.5915.773.50 0.25 34 69.87 1180 50.66 1.41 14.249.41 7.6412.702.420.090.07 0.019 81.9372.265.8718.145.87 0.25 39 64.85 1176 50.70 1.51 14.019.82 7.4312.412.480.090.08 0.015 80.7770.886.1420.508.26 0.25 44 59.83 1171 50.74 1.61 13.7810.27 7.2012.092.550.100.08 0.012 79.4269.386.4022.8410.680.25 49 54.82 1166 50.77 1.73 13.5210.76 6.9411.742.630.110.09 0.010 77.8467.756.6625.1713.110.25 54 49.81 1160 50.79 1.87 13.2511.30 6.6311.342.710.120.10 0.009 75.9765.976.9127.4715.570.25 59 44.80 1152 50.80 2.03 12.9511.89 6.2710.892.810.130.11 0.008 73.7363.997.1529.7418.060.25 64 39.79 1143 50.79 2.23 12.6412.51 5.8410.372.910.150.13 0.008 70.9661.807.3931.9820.600.25 69 34.78 1131 50.76 2.47 12.3113.16 5.319.753.040.170.14 0.008 67.4859.337.6234.1623.200.25 72 32.24 1124 50.73 2.61 12.1513.47 5.009.403.110.190.15 0.009 65.3657.967.7435.2424.530.25 Notes: The relative proportions (in wt. %) of the major element oxides an d crystallizing phases are s hown at ~5 wt. % increment s of crystallization, except when a new mineral comes on the liquidus. Olv = olivine; Plg = plagiocl ase; Cpx – clinopyroxene; Spl = spinel; Fo = forsterite content of crystal lizing olivine; An = anorthite content of crystallizing plagioclase. The oxides and parameters not used in the liquid line of descent modeling have been omitted.

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226Table E-3. 2384-9P at low pressure. Step Melt% T(C) SiO2 TiO2 Al2O3 FeO MgO CaO Na2OK2O P2O5 Cr2O3Fo An Olv% Plg%Cpx%Spl% 1 100.00 1380 49.12 1.01 17.107.04 9.75 11.882.46 0.01 0.07 0.241 -1 -1 0.00 0.000.000.00 2 99.48 1238 49.39 1.01 17.067.34 9.72 11.952.47 0.01 0.07 0.038 88.66-1 0.00 0.000.000.52 3 99.00 1233 49.44 1.02 17.147.32 9.54 12.012.48 0.01 0.07 0.035 88.47-1 0.48 0.000.000.53 5 97.99 1227 49.51 1.03 17.197.33 9.32 12.072.50 0.01 0.07 0.032 88.2080.371.13 0.340.000.54 8 94.98 1225 49.62 1.06 16.977.45 9.20 12.082.53 0.01 0.07 0.033 87.8679.491.95 2.530.000.54 13 89.96 1219 49.82 1.12 16.587.67 8.99 12.112.57 0.02 0.08 0.034 87.2477.943.31 6.200.000.54 18 84.93 1214 50.03 1.19 16.177.90 8.75 12.162.62 0.02 0.08 0.035 86.5476.304.69 9.840.000.54 23 79.91 1207 50.24 1.26 15.718.14 8.49 12.232.66 0.02 0.09 0.036 85.7574.536.06 13.480.000.54 28 74.90 1199 50.47 1.35 15.228.39 8.20 12.332.69 0.02 0.09 0.037 84.8472.647.46 17.100.000.54 33 69.87 1189 50.71 1.44 14.688.66 7.86 12.472.72 0.02 0.10 0.039 83.7770.608.89 20.700.000.54 39 64.85 1179 50.94 1.55 14.158.95 7.51 12.592.76 0.02 0.11 0.039 82.5468.5110.1724.110.330.54 44 59.83 1174 50.97 1.66 13.939.36 7.28 12.292.84 0.02 0.12 0.029 81.3166.9810.4326.472.720.54 49 54.82 1169 50.99 1.79 13.699.81 7.02 11.952.93 0.03 0.13 0.022 79.8965.3010.6728.835.140.54 54 49.81 1163 50.99 1.94 13.4410.32 6.73 11.573.03 0.03 0.14 0.018 78.1963.4610.9131.167.590.54 59 44.80 1155 50.97 2.12 13.1610.87 6.38 11.143.14 0.03 0.16 0.016 76.1661.4111.1333.4710.060.54 64 39.79 1146 50.92 2.34 12.8811.46 5.97 10.643.26 0.04 0.18 0.015 73.6559.1211.3435.7512.580.54 69 34.78 1134 50.84 2.60 12.5812.08 5.46 10.053.42 0.04 0.20 0.015 70.4956.5311.5537.9815.150.54 73 30.99 1123 50.74 2.85 12.3612.53 5.00 9.53 3.55 0.04 0.23 0.015 67.4954.3311.7039.6317.140.54 Notes: The relative proportions (in wt. %) of the major element oxides an d crystallizing phases are s hown at ~5 wt. % increment s of crystallization, except when a new mineral comes on the liquidus. Olv = olivine; Plg = plagiocl ase; Cpx – clinopyroxene; Spl = spinel; Fo = forsterite content of crystal lizing olivine; An = anorthite content of crystallizing plagioclase. The oxides and parameters not used in the liquid line of descent modeling have been omitted.

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227Table E-4. 2384-9P at 2 kbar. Step Melt% T(C) SiO2 TiO2 Al2O3 FeO MgO CaO Na2OK2O P2O5 Cr2O3Fo An Olv%Plg%Cpx%Spl% 1 100.00 1380 49.12 1.01 17.107.04 9.75 11.882.46 0.01 0.07 0.241 -1 -1 0.00 0.000.000.00 2 99.51 1250 49.38 1.01 17.067.31 9.72 11.952.47 0.01 0.07 0.046 88.42-1 0.00 0.000.000.49 3 99.00 1245 49.43 1.02 17.157.30 9.53 12.012.48 0.01 0.07 0.043 88.21-1 0.51 0.000.000.50 5 97.99 1239 49.49 1.03 17.197.31 9.33 12.072.50 0.01 0.07 0.040 87.9676.171.11 0.390.000.51 8 94.98 1237 49.58 1.06 16.987.43 9.22 12.092.51 0.01 0.07 0.041 87.6375.421.91 2.600.000.51 13 89.96 1232 49.73 1.12 16.617.65 9.03 12.152.54 0.02 0.08 0.042 87.0374.103.24 6.290.000.51 18 84.93 1226 49.89 1.19 16.217.88 8.82 12.232.56 0.02 0.08 0.044 86.3572.694.59 9.970.000.51 23 79.91 1220 50.06 1.26 15.778.12 8.58 12.332.58 0.02 0.09 0.046 85.5971.195.95 13.630.000.51 28 74.90 1212 50.23 1.35 15.308.37 8.31 12.462.59 0.02 0.09 0.048 84.7169.587.32 17.280.000.51 32 71.88 1208 50.31 1.40 15.058.55 8.16 12.492.61 0.02 0.10 0.046 84.1568.657.96 19.270.370.51 34 69.87 1207 50.31 1.44 14.978.70 8.10 12.402.63 0.02 0.10 0.041 83.7968.198.08 20.291.250.51 39 64.85 1203 50.30 1.54 14.769.08 7.92 12.172.68 0.02 0.11 0.029 82.7966.978.39 22.803.460.51 44 59.83 1200 50.27 1.65 14.539.52 7.72 11.912.74 0.02 0.12 0.021 81.6565.648.68 25.315.670.51 49 54.82 1195 50.21 1.78 14.2910.00 7.50 11.632.81 0.03 0.13 0.016 80.3364.198.97 27.807.900.51 54 49.81 1190 50.13 1.93 14.0210.54 7.24 11.302.88 0.03 0.14 0.013 78.7762.609.24 30.2810.170.51 59 44.80 1184 50.01 2.11 13.7211.14 6.94 10.942.96 0.03 0.16 0.011 76.9160.839.50 32.7312.460.51 64 39.79 1176 49.84 2.33 13.4011.82 6.59 10.523.05 0.04 0.18 0.010 74.6658.849.75 35.1714.790.51 69 34.78 1166 49.59 2.61 13.0412.58 6.16 10.033.14 0.04 0.20 0.010 71.8556.609.99 37.5617.160.51 74 29.77 1154 49.23 2.96 12.6513.38 5.63 9.45 3.26 0.05 0.24 0.010 68.2854.0210.2239.9119.590.51 79 25.09 1138 48.74 3.39 12.2714.11 5.00 8.79 3.39 0.06 0.28 0.012 63.9351.2310.4142.0521.940.51 Notes: The relative proportions (in wt. %) of the major element oxides an d crystallizing phases are s hown at ~5 wt. % increment s of crystallization, except when a new mineral comes on the liquidus. Olv = olivine; Plg = plagiocl ase; Cpx – clinopyroxene; Spl = spinel; Fo = forsterite content of crystal lizing olivine; An = anorthite content of crystallizing plagioclase. The oxides and parameters not used in the liquid line of descent modeling have been omitted.

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228Table E-5. 2377-7P at low pressure, hydrous conditions. Step Melt% T(C) SiO2 TiO2 Al2O3 FeO MgO CaO Na2OK2O P2O5 H2O Fo An Olv% Plg% Cpx% 1 100.00 1223 48.57 1.23 17.028.03 10.4911.042. 35 0.09 0.14 0.15 88.51-1 0.00 0.00 0.00 2 99.00 1214 48.65 1.24 17.198.01 10.1111.142. 37 0.09 0.14 0.15 88.13-1 1.00 0.00 0.00 6 95.98 1186 48.89 1.28 17.637.95 9.09 11.432. 44 0.09 0.15 0.16 86.9881.813.72 0.30 0.00 7 94.98 1185 48.92 1.29 17.568.00 9.06 11.432. 45 0.10 0.15 0.16 86.8681.523.99 1.03 0.00 12 89.96 1179 49.10 1.37 17.198.23 8.83 11.412. 50 0.10 0.16 0.17 86.1980.055.37 4.67 0.00 17 84.93 1173 49.29 1.45 16.808.47 8.60 11.402. 55 0.11 0.17 0.18 85.4378.476.76 8.30 0.00 22 79.91 1166 49.49 1.54 16.388.72 8.33 11.402. 60 0.11 0.18 0.19 84.5776.798.17 11.910.00 27 74.90 1157 49.70 1.64 15.928.98 8.03 11.432. 64 0.12 0.19 0.20 83.5974.999.59 15.510.00 32 69.87 1147 49.92 1.76 15.429.26 7.69 11.482. 69 0.13 0.20 0.21 82.4573.0411.0319.100.00 37 64.85 1134 50.16 1.89 14.879.53 7.30 11.582. 72 0.14 0.22 0.23 81.0970.9412.5022.660.00 42 59.83 1119 50.43 2.05 14.289.79 6.83 11.722. 76 0.15 0.23 0.25 79.4468.6914.0126.160.00 44 58.83 1116 50.46 2.09 14.209.86 6.75 11.712. 77 0.15 0.24 0.26 79.1068.3014.2126.750.21 48 54.82 1110 50.45 2.22 14.0210.196.52 11.412. 84 0.16 0.26 0.27 77.8267.0114.4028.622.16 53 49.81 1101 50.44 2.40 13.7910.636.17 10.992. 94 0.18 0.28 0.30 75.9365.2714.6630.934.61 58 44.80 1090 50.40 2.62 13.5511.085.78 10.513. 06 0.20 0.31 0.33 73.6763.3514.8933.217.10 63 39.79 1076 50.34 2.89 13.3011.505.30 9.95 3. 19 0.23 0.35 0.38 70.8961.2215.1335.449.64 66 37.13 1067 50.30 3.06 13.1811.695.00 9.62 3. 27 0.24 0.38 0.40 69.1460.0115.2536.6011.02 Notes: The relative proportions (in wt. %) of the major element oxides an d crystallizing phases are s hown at ~5 wt. % increment s of crystallization, except when a new mineral comes on the liquidus. Olv = olivine; Plg = plagiocl ase; Cpx – clinopyroxene; Spl = spinel; Fo = forsterite content of crystal lizing olivine; An = anorthite content of crystallizing plagioclase. The oxides and parameters not used in the liquid line of descent modeling have been omitted.

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229Table E-6. D34-2P at low pressure, hydrous conditions. Step Melt% T(C) SiO2 TiO2 Al2O3 FeO MgO CaO Na2OK2O P2O5 Cr2O3H2O Fo An Olv%Plg%Cpx% 1 100.00 1177 49.53 1.01 16.177.91 9.06 12.882.17 0.06 0.05 0.131 0.14986.93-1 0.000.00 0.00 2 99.00 1165 49.63 1.02 16.347.88 8.68 13.002.19 0.06 0.05 0.132 0.15186.45-1 1.000.00 0.00 4 97.99 1163 49.68 1.03 16.297.91 8.60 13.032.20 0.06 0.05 0.133 0.15286.2880.981.370.64 0.00 7 94.98 1159 49.80 1.07 16.058.03 8.45 13.072.22 0.06 0.05 0.137 0.15785.8580.082.222.80 0.00 12 89.96 1151 50.03 1.13 15.638.23 8.18 13.162.25 0.07 0.06 0.143 0.16685.0678.513.666.38 0.00 18 84.93 1142 50.25 1.19 15.218.45 7.90 13.262.28 0.07 0.06 0.150 0.17584.1776.885.039.87 0.17 23 79.91 1138 50.30 1.26 15.058.75 7.74 13.032.34 0.08 0.06 0.144 0.18683.3475.795.3212.242.52 28 74.90 1134 50.36 1.33 14.889.08 7.56 12.782.40 0.08 0.07 0.140 0.19982.3974.625.6214.614.87 33 69.87 1129 50.40 1.41 14.709.44 7.36 12.502.47 0.09 0.07 0.137 0.21381.3373.365.9016.987.25 38 64.85 1123 50.45 1.49 14.519.83 7.13 12.202.55 0.09 0.08 0.136 0.23080.1072.016.1819.329.65 43 59.83 1117 50.49 1.60 14.3010.25 6.88 11.862.63 0.10 0.08 0.138 0.24978.6970.556.4421.6512.08 48 54.82 1110 50.52 1.71 14.0910.70 6.59 11.482.72 0.11 0.09 0.142 0.27277.0468.976.7023.9514.53 53 49.81 1101 50.54 1.85 13.8611.19 6.26 11.062.82 0.12 0.10 0.149 0.29975.0867.246.9626.2317.01 58 44.80 1091 50.55 2.00 13.6211.69 5.86 10.572.93 0.13 0.11 0.161 0.33372.7265.347.2028.4719.53 63 39.79 1078 50.55 2.19 13.3812.20 5.39 10.013.07 0.15 0.13 0.177 0.37469.8163.227.4430.6822.09 67 36.27 1067 50.55 2.34 13.2212.53 5.00 9.56 3.18 0.16 0.14 0.192 0.41167.3361.607.6232.1923.93 Notes: The relative proportions (in wt. %) of the major element oxides an d crystallizing phases are s hown at ~5 wt. % increment s of crystallization, except when a new mineral comes on the liquidus. Olv = olivine; Plg = plagiocl ase; Cpx – clinopyroxene; Spl = spinel; Fo = forsterite content of crystal lizing olivine; An = anorthite content of crystallizing plagioclase. The oxides and parameters not used in the liquid line of descent modeling have been omitted.

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230Table E-7. 2384-9P at low pressure, hydrous conditions. Step Melt% T(C) SiO2 TiO2 Al2O3 FeO MgO CaO Na2OK2O P2O5 Cr2O3H2O Fo An Olv%Plg%Cpx% 1 100.00 1200 49.07 1.01 17.097.45 9.74 11.872.45 0.01 0.07 0.241 0.15088.54-1 0.000.000.00 2 99.00 1189 49.16 1.02 17.267.42 9.35 11.992.48 0.01 0.07 0.243 0.15288.13-1 1.000.000.00 4 97.99 1181 49.23 1.03 17.367.41 9.07 12.072.50 0.01 0.07 0.245 0.15387.8080.971.780.230.00 7 94.98 1178 49.34 1.06 17.157.53 8.95 12.082.53 0.01 0.07 0.252 0.15887.4480.092.602.420.00 12 89.96 1172 49.52 1.12 16.777.75 8.72 12.102.58 0.02 0.08 0.265 0.16786.7978.563.976.070.00 17 84.93 1164 49.72 1.19 16.367.97 8.47 12.152.62 0.02 0.08 0.280 0.17786.0476.935.369.700.00 22 79.91 1156 49.93 1.27 15.918.20 8.20 12.212.67 0.02 0.09 0.296 0.18885.2075.186.7513.330.00 27 74.90 1146 50.14 1.35 15.448.45 7.88 12.312.71 0.02 0.09 0.315 0.20084.2273.328.1716.930.00 36 66.86 1127 50.50 1.51 14.638.86 7.31 12.492.77 0.02 0.11 0.350 0.22582.3170.1310.3422.500.31 38 64.85 1125 50.50 1.55 14.569.01 7.22 12.372.81 0.02 0.11 0.355 0.23281.8569.5710.4423.441.28 43 59.83 1119 50.51 1.66 14.379.40 6.97 12.042.90 0.02 0.12 0.372 0.25180.5668.0810.6925.793.69 48 54.82 1111 50.51 1.78 14.179.82 6.69 11.683.00 0.03 0.13 0.394 0.27479.0666.4410.9328.126.14 53 49.81 1103 50.48 1.93 13.9610.28 6.37 11.273.11 0.03 0.14 0.425 0.30177.2964.6511.1530.428.62 58 44.80 1092 50.44 2.10 13.7410.76 5.99 10.803.24 0.03 0.16 0.465 0.33575.1462.6811.3732.7011.14 63 39.79 1079 50.35 2.31 13.5211.25 5.54 10.263.40 0.03 0.18 0.518 0.37772.5060.4711.5734.9313.71 68 34.85 1063 50.23 2.56 13.3211.69 5.00 9.62 3.58 0.04 0.20 0.587 0.43169.2258.0411.7637.0816.31 Notes: The relative proportions (in wt. %) of the major element oxides an d crystallizing phases are s hown at ~5 wt. % increment s of crystallization, except when a new mineral comes on the liquidus. Olv = olivine; Plg = plagiocl ase; Cpx – clinopyroxene; Spl = spinel; Fo = forsterite content of crystal lizing olivine; An = anorthite content of crystallizing plagioclase. The oxides and parameters not used in the liquid line of descent modeling have been omitted.

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231Table E-8. 2377-7P at low pr essure using fractionation model of Langmuir (1992). Step Melt% T(C) SiO2 TiO2 Al2O3 FeO MgO CaO Na2OK2O P2O5 Fo An Olv% Plg% Cpx% 1 100.00 1258 48.54 1.22 17.207.92 10.5011.132. 32 0.09 0.13 87.81-1.00 0.00 0.00 0.00 2 99.00 1248 48.62 1.23 17.377.89 10.1211.242. 34 0.09 0.13 87.45-1.00 1.00 0.00 0.00 5 96.99 1232 48.76 1.26 17.617.87 9.53 11.41 2.38 0.09 0.13 86.8281.312.65 0.36 0.00 7 94.98 1231 48.83 1.28 17.497.95 9.43 11.41 2.40 0.10 0.14 86.5681.023.27 1.75 0.00 12 89.96 1226 49.01 1.36 17.168.17 9.17 11.41 2.45 0.10 0.15 85.8780.264.79 5.25 0.00 17 84.93 1220 49.21 1.44 16.818.40 8.88 11.42 2.50 0.11 0.15 85.1079.406.32 8.74 0.00 22 79.91 1214 49.43 1.53 16.428.64 8.57 11.43 2.56 0.11 0.16 84.2278.447.86 12.230.00 27 74.90 1207 49.66 1.63 15.988.90 8.22 11.46 2.61 0.12 0.17 83.2177.359.40 15.700.00 32 69.87 1199 49.93 1.75 15.509.16 7.84 11.50 2.67 0.13 0.19 82.0376.0910.9419.180.00 37 64.85 1190 50.22 1.88 14.959.43 7.42 11.57 2.73 0.14 0.20 80.6574.6312.5022.650.00 42 59.83 1179 50.54 2.04 14.349.70 6.94 11.67 2.79 0.15 0.22 79.0072.9314.0626.100.00 46 56.82 1172 50.73 2.15 13.979.88 6.64 11.71 2.82 0.16 0.23 77.8671.8114.9328.100.15 48 54.82 1170 50.77 2.22 13.8310.046.50 11.58 2.86 0.16 0.24 77.1271.2415.1929.140.85 53 49.81 1163 50.88 2.41 13.4710.496.10 11.21 2.96 0.18 0.26 75.0269.6415.8431.732.62 58 44.80 1154 51.00 2.65 13.0710.955.65 10.79 3.08 0.20 0.29 72.4267.7416.5234.294.40 63 39.79 1144 51.14 2.95 12.6011.425.12 10.30 3.20 0.23 0.33 69.1565.4217.2036.836.18 65 38.75 1142 51.17 3.02 12.5011.515.00 10.18 3.23 0.23 0.34 68.3764.8817.3537.356.55 Notes: The relative proportions (in wt. %) of the major element oxides an d crystallizing phases are s hown at ~5 wt. % increment s of crystallization, except when a new mineral comes on the liquidus. Olv = olivine; Plg = plagiocl ase; Cpx – clinopyroxene; Spl = spinel; Fo = forsterite content of crystal lizing olivine; An = anorthite content of crystallizing plagioclase. The oxides and parameters not used in the liquid line of descent modeling have been omitted.

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232Table E-9. D34-2P at low pressure usi ng fractionation model of Langmuir (1992). Step Melt% T(C) SiO2 TiO2 Al2O3 FeO MgO CaO Na2OK2O P2O5 Cr2O3Fo An Olv%Plg%Cpx%Spl% 1 100.00 1297 49.59 1.02 16.197.64 9.07 12.892.17 0.06 0.05 0.131 -1 -1 0.00 0.00 0.00 0.00 2 99.76 1213 49.72 1.02 16.177.82 9.06 12.922.17 0.06 0.05 0.040 86.25-1 0.00 0.00 0.00 0.24 3 99.17 1207 49.78 1.02 16.267.80 8.84 13.002.19 0.06 0.05 0.037 86.0080.420.58 0.00 0.00 0.25 4 99.00 1207 49.79 1.02 16.257.80 8.83 13.002.19 0.06 0.05 0.037 85.9780.390.63 0.13 0.00 0.25 8 94.98 1202 49.97 1.07 15.967.96 8.61 13.072.21 0.06 0.05 0.039 85.4079.751.85 2.92 0.00 0.25 13 89.96 1196 50.21 1.13 15.558.15 8.32 13.182.25 0.07 0.06 0.038 84.6178.853.37 6.41 0.00 0.26 16 87.95 1194 50.28 1.15 15.428.24 8.21 13.182.26 0.07 0.06 0.038 84.2878.513.86 7.70 0.23 0.26 19 84.93 1192 50.32 1.19 15.318.41 8.10 13.082.30 0.07 0.06 0.040 83.8078.114.23 9.29 1.29 0.26 24 79.91 1189 50.40 1.25 15.118.71 7.89 12.912.35 0.08 0.06 0.039 82.9277.404.85 11.933.04 0.27 29 74.90 1186 50.49 1.33 14.899.03 7.66 12.712.41 0.08 0.07 0.042 81.9376.625.47 14.574.79 0.27 34 69.87 1182 50.58 1.41 14.659.38 7.42 12.492.48 0.09 0.07 0.041 80.8075.736.09 17.216.55 0.27 39 64.85 1178 50.69 1.51 14.399.76 7.14 12.252.55 0.09 0.08 0.041 79.4974.736.73 19.838.31 0.28 44 59.83 1173 50.80 1.63 14.1010.17 6.83 11.982.63 0.10 0.08 0.044 77.9773.597.38 22.4410.070.28 49 54.82 1167 50.93 1.76 13.7710.61 6.48 11.662.71 0.11 0.09 0.044 76.1672.298.03 25.0211.840.29 54 49.81 1161 51.08 1.92 13.4111.09 6.09 11.312.81 0.12 0.10 0.044 74.0070.758.70 27.5913.610.29 59 44.80 1152 51.25 2.11 13.0011.59 5.63 10.892.91 0.13 0.11 0.045 71.3468.929.38 30.1415.380.30 65 38.81 1140 51.50 2.40 12.4212.20 5.00 10.293.05 0.15 0.13 0.043 67.2766.2210.2233.1517.520.31 Notes: The relative proportions (in wt. %) of the major element oxides an d crystallizing phases are s hown at ~5 wt. % increment s of crystallization, except when a new mineral comes on the liquidus. Olv = olivine; Plg = plagiocl ase; Cpx – clinopyroxene; Spl = spinel; Fo = forsterite content of crystal lizing olivine; An = anorthite content of crystallizing plagioclase. The oxides and parameters not used in the liquid line of descent modeling have been omitted.

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233Table E-10. 2384-9P at low pressure usi ng fractionation models of Langmuir (1992). Step Melt% T(C) SiO2 TiO2 Al2O3 FeO MgO CaO Na2OK2O P2O5 Cr2O3Fo An Olv%Plg%Cpx%Spl% 1 100.00 1380 49.12 1.01 17.107.04 9. 75 11.882.46 0.01 0.07 0.241-1 .00-1.000.000.00 0.00 0.00 2 99.46 1233 49.40 1.01 17.057.35 9. 72 11.952.47 0.01 0.07 0.03387. 79-1.000.000.00 0.00 0.54 4 98.65 1224 49.49 1.02 17.197.32 9.41 12.052.49 0.01 0.07 0.028 87.4980.140.790.00 0.00 0.56 5 97.99 1223 49.51 1.03 17.147.34 9.38 12.062.50 0.01 0.07 0.028 87.4180.040.990.46 0.00 0.56 8 94.98 1220 49.63 1.06 16.957.46 9.23 12.082.52 0.01 0.07 0.029 87.0479.571.902.57 0.00 0.56 13 89.96 1215 49.84 1.12 16.607.66 8.95 12.122.57 0.02 0.08 0.031 86.3678.723.416.07 0.00 0.56 18 84.93 1209 50.07 1.19 16.227.87 8.65 12.182.63 0.02 0.08 0.033 85.5877.764.939.58 0.00 0.56 23 79.91 1202 50.32 1.26 15.798.09 8.33 12.252.68 0.02 0.09 0.035 84.7176.676.4513.080.00 0.56 28 74.90 1194 50.59 1.35 15.338.32 7.97 12.352.73 0.02 0.09 0.037 83.6975.447.9716.570.00 0.56 33 69.89 1185 50.89 1.44 14.818.56 7.57 12.472.78 0.02 0.10 0.040 82.5074.039.5120.050.00 0.56 34 69.87 1185 50.89 1.44 14.818.56 7. 57 12.472.78 0.02 0.10 0.040-1 .0074.039.5120.060.01 0.56 39 64.85 1180 50.99 1.54 14.578.91 7.30 12.232.87 0.02 0.11 0.039 81.3072.9510.1322.681.78 0.57 44 59.83 1176 51.10 1.66 14.309.30 7.00 11.952.96 0.02 0.12 0.039 79.9071.7310.7525.303.55 0.57 49 54.82 1170 51.22 1.79 14.009.71 6.65 11.643.07 0.03 0.13 0.039 78.2470.3211.3927.895.32 0.58 54 49.81 1163 51.35 1.95 13.6710.16 6.27 11.283.19 0.03 0.14 0.039 76.2468.6612.0230.487.11 0.58 59 44.80 1155 51.50 2.15 13.3010.65 5.82 10.863.32 0.03 0.16 0.040 73.7966.6812.6833.048.90 0.59 64 39.79 1145 51.67 2.39 12.8811.15 5.31 10.363.46 0.04 0.18 0.041 70.7064.2813.3435.5810.700.59 67 37.17 1138 51.77 2.54 12.6311.41 5.00 10.073.55 0.04 0.19 0.039 68.7462.8013.6936.8911.650.60 Notes: The relative proportions (in wt. %) of the major element oxides an d crystallizing phases are s hown at ~5 wt. % increment s of crystallization, except when a new mineral comes on the liquidus. Olv = olivine; Plg = plagiocl ase; Cpx – clinopyroxene; Spl = spinel; Fo = forsterite content of crystal lizing olivine; An = anorthite content of crystallizing plagioclase. The oxides and parameters not used in the liquid line of descent modeling have been omitted.

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234Table E-11. 2384-9P at 2 kbar using fr actionation models of Langmuir (1992). Step Melt% T(C) SiO2 TiO2 Al2O3 FeO MgO CaO Na2OK2O P2O5 Cr2O3Fo An Olv%Plg%Cpx%Spl% 1 100.00 1380 49.12 1.01 17.107.04 9. 75 11.882.46 0.01 0.07 0.241-1. 00 -1.00 0.00 0.00 0.00 0.00 2 99.49 1242 49.39 1.01 17.067.33 9. 72 11.952.47 0.01 0.07 0.04087. 79-1.00 0.00 0.00 0.00 0.51 3 99.00 1236 49.44 1.02 17.147.31 9. 54 12.012.48 0.01 0.07 0.03887. 61-1.00 0.49 0.00 0.00 0.52 5 97.99 1230 49.51 1.03 17.207.31 9.31 12.082.50 0.01 0.07 0.033 87.3479.031.16 0.31 0.00 0.54 8 94.98 1227 49.62 1.06 17.017.43 9.16 12.102.52 0.01 0.07 0.034 86.9778.562.04 2.44 0.00 0.54 13 89.96 1222 49.82 1.12 16.667.63 8.89 12.152.57 0.02 0.08 0.036 86.3077.713.53 5.97 0.00 0.54 18 84.93 1216 50.04 1.19 16.277.84 8.61 12.222.61 0.02 0.08 0.038 85.5376.755.03 9.50 0.00 0.54 23 79.91 1209 50.27 1.26 15.858.07 8.29 12.302.66 0.02 0.09 0.041 84.6675.686.52 13.030.00 0.54 28 75.90 1204 50.44 1.33 15.528.26 8.04 12.342.70 0.02 0.09 0.043 83.8974.787.60 15.720.24 0.54 29 74.90 1203 50.45 1.34 15.488.33 7.99 12.302.71 0.02 0.09 0.043 83.7074.617.71 16.260.59 0.54 34 69.87 1200 50.52 1.43 15.258.66 7.76 12.092.79 0.02 0.10 0.043 82.6973.688.30 18.942.34 0.55 39 64.85 1196 50.60 1.52 15.019.03 7.50 11.862.87 0.02 0.11 0.043 81.5272.658.90 21.604.10 0.55 44 59.83 1191 50.68 1.64 14.749.43 7.20 11.612.96 0.02 0.12 0.043 80.1671.479.51 24.255.86 0.56 49 54.82 1186 50.76 1.77 14.449.87 6.87 11.313.06 0.03 0.13 0.043 78.5570.1110.1226.877.62 0.56 54 49.81 1180 50.85 1.92 14.1110.35 6.50 10.983.17 0.03 0.14 0.044 76.6268.5310.7429.499.39 0.57 59 44.80 1173 50.94 2.11 13.7310.87 6.07 10.593.29 0.03 0.16 0.045 74.2666.6611.3732.0911.180.57 64 39.79 1163 51.05 2.34 13.3111.43 5.57 10.133.43 0.04 0.18 0.042 71.3064.4012.0034.6512.970.58 69 34.89 1152 51.15 2.62 12.8311.99 5.00 9.59 3.58 0.04 0.20 0.044 67.6061.6812.6437.1314.750.59 Notes: The relative proportions (in wt. %) of the major element oxides an d crystallizing phases are s hown at ~5 wt. % increment s of crystallization, except when a new mineral comes on the liquidus. Olv = olivine; Plg = plagiocl ase; Cpx – clinopyroxene; Spl = spinel; Fo = forsterite content of crystal lizing olivine; An = anorthite content of crystallizing plagioclase. The oxides and parameters not used in the liquid line of descent modeling have been omitted.

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235Table E-12. D20-15P at low pressure. Step Melt% T(C) SiO2 TiO2 Al2O3 FeO MgOCaO Na2OK2OP2O5Cr2O3 H2OFo An Olv%Plg%Cpx%Spl% 1 100.00 1353 48.98 0.94 17.41 6.97 9. 84 12.03 2.44 0 0.07 0.175 0 -1 .00-1.000.000.00 0.00 0.00 2 99.61 1240 49.19 0.94 17.37 7.20 9. 82 12.08 2.45 0 0.07 0.035 0 88.99-1.000.000.00 0.00 0.39 3 99.00 1234 49.24 0.94 17.47 7.19 9. 58 12.15 2.46 0 0.07 0.032 0 88.75-1.000.610.00 0.00 0.40 5 97.99 1230 49.30 0.95 17.46 7.21 9.45 12.19 2.48 0 0.07 0.03 0 88.5781.301.060.54 0.00 0.41 10 92.98 1226 49.48 1.00 17.10 7.42 9.26 12.20 2.53 0 0.08 0.03 0 88.0179.832.424.20 0.00 0.41 15 87.95 1221 49.68 1.06 16.70 7.64 9.05 12.24 2.57 0 0.08 0.031 0 87.3978.243.777.87 0.00 0.41 20 82.93 1215 49.88 1.12 16.28 7.88 8.81 12.29 2.62 0 0.08 0.032 0 86.6976.565.1511.520.00 0.41 25 77.91 1208 50.10 1.20 15.81 8.13 8.55 12.37 2.66 0 0.09 0.033 0 85.8974.766.5215.160.00 0.41 30 72.89 1200 50.32 1.28 15.31 8.39 8.25 12.48 2.70 0 0.10 0.034 0 84.9772.827.9218.780.00 0.41 35 67.86 1190 50.56 1.37 14.76 8.67 7.91 12.63 2.74 0 0.10 0.036 0 83.8970.729.3422.390.00 0.41 40 63.62 1180 50.78 1.47 14.26 8.91 7.58 12.80 2.76 0 0.11 0.037 0 82.8168.8310.5625.410.00 0.41 41 62.84 1180 50.78 1.48 14.22 8.98 7.55 12.76 2.77 0 0.11 0.035 0 82.6368.5910.6025.780.37 0.41 45 58.83 1176 50.80 1.57 14.05 9.32 7.37 12.52 2.84 0 0.12 0.027 0 81.6567.3410.8127.672.28 0.41 50 53.81 1171 50.81 1.69 13.81 9.79 7.12 12.19 2.93 0 0.13 0.021 0 80.2465.6411.0630.034.69 0.41 55 48.81 1165 50.79 1.83 13.56 10.30 6.82 11.81 3.03 0 0.14 0.017 0 78.5563.7711.3032.377.12 0.41 60 43.80 1158 50.76 2.00 13.28 10.88 6.48 11.39 3.15 0 0.16 0.014 0 76.5261.6911.5434.699.57 0.41 65 38.78 1149 50.69 2.21 12.99 11.51 6.07 10.90 3.28 0 0.18 0.013 0 74.0259.3411.7636.9812.070.41 70 33.77 1137 50.57 2.47 12.69 12.18 5.57 10.32 3.44 0 0.21 0.013 0 70.8456.6711.9839.2214.620.41 75 29.23 1123 50.40 2.76 12.42 12.78 5.00 9.70 3.62 0 0.24 0.014 0 67.1453.9212.1641.2116.990.41 Notes: The relative proportions (in wt. %) of the major element oxides an d crystallizing phases are s hown at ~5 wt. % increment s of crystallization, except when a new mineral comes on the liquidus. Olv = olivine; Plg = plagiocl ase; Cpx – clinopyroxene; Spl = spinel; Fo = forsterite content of crystal lizing olivine; An = anorthite content of crystallizing plagioclase. The oxides and parameters not used in the liquid line of descent modeling have been omitted.

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236Table E-13. 2375-7P at low pressure. Step Melt% T(C) SiO2 TiO2 Al2O3 FeO MnOMgOCaO Na2OK2O P2O5H2OFo An Olv%Plg%Cpx% 1 100.0 1204 50.22 1.38 15.32 8.63 0.17 8.29 12.02 2.72 0.09 0.11 0 84.66-1.000.000.00 0.00 2 99.8 1202 50.24 1.39 15.35 8.62 0.17 8.21 12.05 2.73 0.09 0.11 0 84.5472.730.210.00 0.00 3 99.0 1201 50.27 1.40 15.30 8.65 0.17 8.17 12.06 2.73 0.09 0.11 0 84.4272.500.440.57 0.00 5 97.0 1198 50.34 1.43 15.14 8.73 0.17 8.08 12.09 2.75 0.09 0.11 0 84.1271.901.002.02 0.00 10 92.0 1191 50.51 1.50 14.73 8.94 0.18 7.82 12.19 2.77 0.09 0.12 0 83.2770.352.425.61 0.00 15 86.9 1182 50.69 1.59 14.29 9.16 0.18 7.54 12.32 2.79 0.10 0.12 0 82.3168.693.879.18 0.00 20 82.9 1175 50.83 1.67 13.95 9.33 0.19 7.30 12.41 2.81 0.10 0.13 0 81.4567.364.9411.910.22 25 77.9 1171 50.84 1.76 13.78 9.66 0.19 7.12 12.17 2.87 0.11 0.14 0 80.4666.175.1914.282.62 30 72.9 1167 50.83 1.86 13.60 10.00 0.20 6.92 11.91 2.94 0.12 0.15 0 79.3464.895.4316.635.05 35 67.9 1162 50.82 1.97 13.41 10.38 0.21 6.69 11.63 3.01 0.13 0.16 0 78.0663.525.6718.977.50 40 62.8 1157 50.80 2.10 13.21 10.78 0.21 6.45 11.32 3.09 0.14 0.17 0 76.6062.045.9021.299.96 45 57.8 1151 50.75 2.25 13.00 11.20 0.22 6.16 10.97 3.18 0.15 0.18 0 74.9160.446.1323.5912.46 50 52.8 1143 50.69 2.43 12.78 11.64 0.23 5.84 10.59 3.27 0.16 0.20 0 72.9258.706.3525.8614.98 55 47.8 1135 50.61 2.63 12.56 12.09 0.24 5.46 10.16 3.38 0.18 0.22 0 70.5756.786.5628.1017.54 60 42.8 1124 50.49 2.87 12.34 12.53 0.25 5.02 9.67 3.51 0.20 0.25 0 67.7354.686.7730.2920.15 Notes: The relative proportions (in wt. %) of the major element oxides an d crystallizing phases are s hown at ~5 wt. % increment s of crystallization, except when a new mineral comes on the liquidus. Olv = olivine; Plg = plagiocl ase; Cpx – clinopyroxene; Spl = spinel; Fo = forsterite content of crystal lizing olivine; An = anorthite content of crystallizing plagioclase. The oxides and parameters not used in the liquid line of descent modeling have been omitted.

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237 APPENDIX F REGRESSION ANALYSIS FOR FE8.0 AND NA8.0 Na2O and FeO values can be “corrected” for low-pressure fractionation by extrapolating the Na2O and FeO values along a line of constant slope to 8 wt% MgO. The line is meant to approximate the slope of the liquid lines of decent (LLD), which approximates the change in melt composition pr oduced during crystall ization and will fit samples related by fractionation. Samples fo r individual regions usually form rather smooth trends on plots of oxide abundances as a function of MgO. The best-fit line is meant to approximate the slope of the olivine+plagioclasec linopyroxene LLD. The problem with using just one line is that there are strong kinks in the slope of the data and the LLD when a new mineral joins the fractio nating assemblage. In the Siqueiros data, there is a kink when plagioclase joins th e olivine and when clinopyroxene joins the assemblage. In order to fit th e trend of the data, two lines of different slope were used to match the kinks in the data (Figures D-1 and D-2). This allowed the samples with higher MgO values to be projected along the shallow olivine slope and the samples with lower MgO along the steeper olivine-plagioclase sl ope. For this data set, two lines fit reasonably well, but a better fit was obt ained by using a second order polynomial (Figures D-3 and D-4). The polynomial was better at fitting th e kinks because the Siqueiros data appears to be best explaine d by more than one LLD and the kinks do not appear to be at the same MgO for each LLD making it difficult to choose two lines to fit the data. The curve of the polynomial was be tter at fitting all of the data. The R2 was

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238 0.95 for FeO vs. MgO and 0.77 for Na2O vs. MgO. The equation of the polynomial was then used to extrapolate the data back to 8wt% MgO. The calculated values at 8 wt% MgO of Na2O and FeO are called Na8.0 and Fe8.0, respectively.

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239 Figure F-1. Linear regression of Na2O. Two best fit-lines used to match the shallow olivine slope and the steepe r olivine plagioclase clinopyroxene slope. The kink in the slope set at 9.0 wt% MgO based on the calculated LLDs in chapter 6. A best-fit 2nd order polynomial was us ed instead to extrapolate data back to 8 wt% MgO. Figure F-2. Linear regression of FeO. Two best fit-lines used to match the shallow olivine slope and the steepe r olivine plagioclase clinopyroxene slope. The kink in the slope set at 9.98 wt% MgO based on the calculated LLDs in Chapter 6. A best-fit 2nd order polynom ial was used instead to extrapolate data back to 8 wt% MgO.

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240 2 2.2 2.4 2.6 2.8 3 3.2 5678910111213Na2OMgO Y = M0 + M1*x + ... M8*x8 + M9*x9 5.6741 M0 -0.59924 M1 0.027111 M2 0.76946 R Figure F-3. Polynomial regression of Na2O. The best-fit 2nd order polynomial that was used to extrapolate Na2O data back to 8 wt% MgO is shown. Na8.0 = Na2O + -0.59924 (8 – MgO) + 0.027111 (64 – MgO2). 7 8 9 10 11 12 13 14 15 5678910111213FeOTMgO Y = M0 + M1*x + ... M8*x8 + M9*x9 27.336 M0 -3.2233 M1 0.12974 M2 0.95113 R Figure F-4. Polynomial regression of FeO. The best-fit 2nd order polynomial that was used to extrapolate FeO data b ack to 8 wt% MgO is shown. Fe8.0 = FeO + 3.2233 (8 – MgO) + 0.12974 (64MgO2)

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251 BIIOGRAPHICAL SKETCH Michelle R. Hays earned her Bachelor of Science degree in environmental studiesearth science from the University of Nebr aska at Omaha in 2001. She began graduate studies in geology at the Universi ty of Florida in the fall of 2001.


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Title: Intra-Transform Volcanism along the Siqueiros Fracture Zone 8 Degrees 20 Minutes N to 8 Degrees 30 Minutes N, East Pacific Rise
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Copyright Date: 2008

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INTRA-TRANSFORM VOLCANISM ALONG THE SIQUEIROS FRACTURE ZONE
8020' N 8030' N, EAST PACIFIC RISE















By

MICHELLE RENAE HAYS


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2004















ACKNOWLEDGMENTS

Foremost, I thank Dr. Michael Perfit for all his guidance and support throughout

this project. I thank my committee members, Dr. Paul Mueller and Dr. David Foster, for

their time and discussion. I give special thanks to Dr. Daniel Fomari, Dr. Ian Ridley, the

captain and crew of the Atlantis II, and the Alvin pilots for this project would not have

been possible without their efforts. I especially thank Dr. Daniel Fornari and Dr. Ken

Simms for their early guidance in my graduate studies. I thank Dr. Leonard

Danyushevsky for the Cameca microprobe analysis completed at the University of

Tasmania, Dr. Ian Jonasson for the ICP-ASE data from the Geological Survey of Canada,

and Dr. Jack Casey of the University of Houston for ICP-MS analysis. I would also like

to thank Dr. Robert Shuster and Dr. Harmon Maher from the University of Nebraska for

their encouragement and inspiration. Most of all, I thank Troy and the rest of my family

for their love, support and everlasting patience. I could not have made it this far without

them. This work was supported by The National Science Foundation through grants

OCE-90-19154, OCE-90-20404, and OCE-0138088 and by the University of Florida

through the Alumni Fellowship.
















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S .................................................................................................. ii

LIST OF TABLES.. .. ....... ......... ................... .................v

LIST OF FIGURES ........... ......... .............. ........... vii

ABSTRACT................................. .............. xii

CHAPTER

1 INTRODUCTION ................... ...................................... ......... .......

Sam pling of the Siqueiros Transform .........................................................................3
The Transform Fault Effect ............................................... ...............6
Mantle Heterogenities.................................8

2 GEOLOGICAL RELATIONSHIPS AND SAMPLE LOCALITIES IN THE
SIQUEIROS TRANSFORM....................... ............. ...............10

3 A N A L Y TIC A L M E TH O D S ................................................................................. 20

4 PETROGRAPHY AND LOCAL GEOLOGIC RELATIONSHIPS..........................28

Sam ples from the A -B F ault .................................................................................. 28
Siqueiros Sample Petrography......................... ......................42
Crystal Liquid Equilibria ...... ................... ...............44

5 MAJOR AND TRACE ELEMENT CHEMISTRY ...................................... 61

M ajor Elem ent Trends .............. ....... ..6.. .. .... .... .. .. .. .. ..........61
Comparison of Siqueiros Samples to the Adjacent EPR and Garrett Transform .......73
Trace Element Trends... .................. .............................. 79

6 PETROGENESIS .................................................. ..... ..............104

M ajor Elem ent M odels ................................. .......................................................104
Trace Element Models ....................... ... ............... 119
REE Models ..................... ... ...... .......... 133









7 DISCUSSION ................ ...................... ..................... ...... .............. 138

Fractional Crystallization.................................. 138
M agm a M ixing and A ssim ilation .................................................................... 140
D-MORBs and E-MORBs................ ........ ............. ...............148
Controls on Spatial Variability in Lava Chemistry .................................. 150
Tectonic Controls on Magmagenesis and Melting Systematics.....................1...56
Constraints on M elting -N a-Fe System atics.....................................................159
Models for Volcanism in Transform Domains............... ................. ........168
Garrett Transform M odels ...................................................... 170
Siqueiros Transform M models ...................................................................... 174
Proposed M odel ............... ...................... ...... ......... ...... .... ............... 176

8 CONCLUSIONS ................................................... .180

APPENDIX

A NORMALIZATION OF CAMECA MICROPROBE DATA ...............................182

B OLIVINE, PLAGIOCLASE AND SPINEL MICROPROBE ANALYSIS............185

C MAJOR ELEMENT COMPOSITIONS OF THE SIQUEIROS SAMPLES.......199

D TRACE ELEMENT CONTENTS OF THE SIQUEIROS SAMPLES....................209

E FRACTIONAL CRYSTALLIZATION MODEL PARAMETERS
CALCULATED IN PETROLOG.... ................... .............. 223

F REGRESSION ANALYSIS FOR FE8.o AND NA8.o ..........................................237

G L IST O F R E FE R E N C E S ..................................................................................... 24 1

H BIIOGRAPHICAL SKETCH...................... ...........................251
















LIST OF TABLES


Table page

2-1 Siqueiros transform Alvin dive locations.............................................................. 15

2-2 Siqueiros transform dredge locations. ............................... .. ......16

4-1 Thin section descriptions................................................ 29

5-1 Nb, Sr, Zr, and Y enrichment factors for Siqueiros transform morphotectonic
locations. ........................................................86

6-1 List of partition coefficients. ................ ................... ....... .... ...........120

6-2 REE partition coefficients. ............................................. ............... 134

B-1 Microprobe analysis of olivine phenocrysts in the Siqueiros samples................1...86

B-2 Microprobe analysis of plagioclase phenocrysts in the Siqueiros samples..........190

B-3 Microprobe analysis of spinel phenocrysts in the Siqueiros samples...................195

C-i ARL, JEOL, and DCP electron microprobe major element analyses of basalts
from the Siqueiros transform........................ ......... ......... 200

C-2 Siqueiros glass major element analysis. ..................... .................205

D-1 XRF trace element concentrations for the Siqueiros transform basalts. ..............210

D-2 ICP Trace element concentrations for the Siqueiros transform basalts................214

D-3 DCP trace element concentrations for the Siqueiros transform basalts. ..............217

D-4 ICP trace element concentrations of the Siqueiros transform basalts....................218

E-i 2377-7P at low pressure. ........................................224

E-2 D 34-2P at low pressure. .............................................. ............... 225

E-3 2384-9P at low pressure. ............................................................... .............226

E-4 2384-9P at 2 kbar. ....................... .................... ..............227










E-5 2377-7P at low pressure, hydrous conditions....................................228

E-6 D34-2P at low pressure, hydrous conditions..........................................................229

E-7 2384-9P at low pressure, hydrous conditions................... ................230

E-8 2377-7P at low pressure using fractionation model of Langmuir (1992). .............231

E-9 D34-2P at low pressure using fractionation model of Langmuir (1992). ..............232

E-10 2384-9P at low pressure using fractionation models of Langmuir (1992).............233

E-l 1 2384-9P at 2 kbar using fractionation models of Langmuir (1992). ....................234

E-12 D20-15P at low pressure. ..............................................235

E-13 2375-7P at low pressure. .............................................. ............... 236
















LIST OF FIGURES


Figure page

1-1 Location map of Siqueiros transform. ............................................2

1-2 Sample location map. ......................................... ...... ..... .. .5

2-1 Plate boundary geometry of the Siqueiros transform......... ...................11

2-2 Bathymetry and sample locations for west side of the Siqueiros transform. .........18

2-3 Bathymetry and sample locations for east side of the Siqueiros transform. ............19

3-1 Graphical comparison of ARL microprobe, JEOL microprobe, DCP, and
Cameca SX50 data before correction of the data..............................................23

3-2 Comparison of data after adjustment of the Cameca SX50 MgO and P205
contents................... .. ................................ ......... 25

4-1 Photomicrographs taken under plain light (a & b) and cross polarized light
(c & d) .............................. ................... ........ 36

4-2 Photomicrographs taken under plain light (a, b, & d) and cross polarized light
(c) .............................. ................. ........ 37

4-3 Photomicrographs taken under plain light (c) and cross polarized light
(a, b, & d). .........................................................3 8

4-4 Dredge and Alvin dive locations within the A-B fault..........................................40

4-5 A lvin div e 2384 trav erse. ................................................................................... 4 1

4-6 Comparison of olivine forsterite content with the Mg# (Mg2+/(Mg2+ + Fe2+))
of the host glass. ........................ ....... ................... .......... ......... 45

4-7 Comparison of Olivine forsterite content with the Mg# (Mg2+/(Mg2+ + Fe2+))
of the host glass. ........................ ....... ................... .......... ......... 47

4-8 Calculated Fo contents of olivine for partition coefficients ranging from 0.28 to
0.32 .............................. .................... ........ 48









4-9 An contents for core, interior, and rim locations in Siqueiros plagioclase
phenocrysts ................. .... ................ ......... .49

4-10 Comparison of plagioclase An content from Siqueiros samples and An content
evolution for three of the major element parental compositions..............................50

4-11 Comparison of the host glass Ca# (100*Ca/(Ca + Na) with the plagioclase An
content. ........................................................52

4-12 Comparison of Siqueiros plagioclase An content vs. glass Ca#
(100*Ca/(Ca +Na)). ............................................ ....... ........ 53

4-13 Spinel Cr# for core, interior, and rim locations.......................... ...........54

4-14 Fe3+/(Cr + Al + Fe3+) vs. Fe2+/(Mg + Fe2+) plots for tholeiitic basalts..................55

4-15 Cr/(Cr + Al) vs. Fe2+/(Mg + Fe2+) plot for tholeiitic basalts..............................56

4-16 Molecular percentage aluminum in glass versus molecular percentage aluminum
in spinel. .........................................................58

4-17 Comparison of the composition of the cores, interiors, and rims of spinels
found in the groundmasses and within olivines with the composition of the host
glass.. ..................................................58

4-18 Comparison of the composition of the cores, interiors, and rims of spinels
found in the groundmasses and within olivines with the composition of the host
glass.......................................... .........59

4-19 Comparison of the composition of the spinels found inside olivines and spinels
found in the glass with the composition of the host glass..................... ..........59

4-20 Comparison of the composition of the spinels found inside olivines and spinels
found in the glass with the composition of the host glass.......................................60

5-1 Major element variation diagrams for glasses from the Siqueiros transform
dom ain............................................................... 62

5-2 Major element variation diagrams showing the Siqueiros picrites and picritic
basalts relative to more evolved MORB as in Figure 5-1.............. ...............65

5-3 Comparison of K20/TiO2 of Siqueiros samples with samples from the EPR.......... 69

5-4 MgO (wt. %) and depth to seafloor versus distance from the axis of spreading
center B .. .............. .......... ............. ........ 74

5-5 Variation diagrams comparing Siqueiros lava compositions with basalts from
the 9-100N segment of the EPR. ........... ........ ..................75









5-6 Variation diagrams comparing the compositions of the Siqueiros and Garrett
samples ...................................... .................................. ......... 77

5-7 Trace elem ents versus TiO2........................................................... 80

5-8 Trace elements versus Zr..... ............ ........ ..... ........83

5-9 Ce/Ybn vs. K20/TiO2 of the Siqueiros samples. .......................................... 88

5-10 Chondrite normalized Ce/Yb ratios for Siqueiros morphotectonic locations. .........89

5-11 Chondrite normalized REE diagrams.............................. ............... 91

5-12 N-Morb normalized REE diagrams........................................ 95

5-13 N-MORB normalized Ce/Y ratios for Siqueiros transform morphotectonic
locations.. ........................... ................... ......... 100

5-14 REE diagram of RTI E-MORBs plotted relative to E-MORB values.. ...............100

5-15 Primitive mantle-normalized trace element diagrams................ ....101

6-1 Percentage of crystals removed as a function of temperature............... ...............106

6-2 Percentage of liquid and removed crystals as a function of percentage of
crystals removed from magma for 2377P, D34-2P and 2384-9P .............107

6-3 Comparison of major element data with LLD models calculated using the
olivine, plagioclase, and clinopyroxene fractionation models of
Danyushevsky. .................................................... 109

6-4 Comparison of major element data with hydrous LLD models calculated using
the olivine, plagioclase and clinopyroxene fractionation models of
Danyushevsky...................................... .............................. ......... 113

6-5 Comparison of CaO and A1203 data with LLD models calculated using the
olivine, plagioclase, and clinopyroxene fractionation models of Langmuir et al.. 118

6-6 Comparison of observed trace element data with modeled fractionation trends
calculated assuming perfect Rayleigh fractional crystallization.........................121

6-7 Comparison of observed trace element data versus TiO2 with modeled
fractionation trends calculated assuming perfect Rayleigh fractional
crystallization. ............ ............ ......................... .............124

6-8 Comparison of observed trace element data versus TiO2 with modeled
fractionation trends calculated assuming perfect Rayleigh fractional
crystallization ....................................................129









6-9 Comparison of observed trace element data versus Zr with modeled
fractionation trends calculated assuming perfect Rayleigh fractional
crystallization. ........................... ............... ........ 130

6-10 Comparison of observed REE trends with modeled REE fractionation trends
calculated for 2375-7P from spreading center B...................................................134

6-11 Comparison of observed REE trends with modeled REE fractionation trends
calcu ated for D20-15P from the A-B fault.....................................................135

6-12 Comparison of observed A-B fault REE trends with modeled REE fractionation
trends calculated for D20-15P......... ................................................. ....... 135

6-13 Rayleigh fractionation model for REE ....................................137

7-1 Mixing lines between primitive and evolved sample compositions from the
Siqueiros transform. .................. ............ .... ...... .... 144

7-2 Trace element mixing lines between primitive and evolved samples..................146

7-3 Calculated mixing curves between sample 2384-9 and an evolved sample from
spreading center B (2377-11) and an E-MORB from the RTI (23 90-1).............1...47

7-4 Chondrite and N-MORB normalized Ce/Y ratios for Siqueiros transform
m orphotectonic locations. ............................................ ............... 149

7-5 Chondrite normalized La/Sm ratios for Siqueiros transform morphotectonic
locations. ........................................................151

7-6 Calculated mixing between sample D20-15 (D-MORB compositions) and
sample 2390-1 (E-MORB composition) .........................................152

7-7 LLD for 2384-9P after mixing with 10% E-MORB ........................ .........153

7-8 Modeled fractional crystallization path of 6% mixing line from figure 7-6. .......154

7-9 Location map of E-MORB, N-MORB, and D-MORB samples within the
Siqueiros transform based on Ce/Y ratios............. ..... ..................155

7-10 Position of "apparent" Euler poles associated with a counterclockwise change
in spreading direction along the Clipperton and Siqueiros Fracture Zones. ........157

7-11 Siqueiros Nas.o and Fes.o data compared with global field for normal ridge
segments. ....................................................... 162

7-12 Nas.o vs. Fes.o. .................................................................163

7-13 N as.o vs. Feso and K20/TiO2. ......................................................165


........................................................................................... .. 1 6 2

7-12 N as.o vs. F e .o. ......................................................... ...................... .. ..... 163

7-13 N a .0o vs. Fe8.0 and K 20/TiO 2. ......................................................... 165









7-14 Ce/Y ratios vs. Nas.o values for all Siqueiros transform samples.......................166

7-15 Ce/Y ratios vs. Na8.o values for samples from spreading center A. ....................166

7-16 K20/TiO2 ratios of Siqueiros samples compared with their Na8.o, Fe8.o data.........167

7-17 Fe8.o versus Na8.o and axial depth....................... ........ .... ......167

7-18 Variations in Nas.o and Fes.o systematics due to variable depths and extents of
melting........................ ..... ................ 168

7-19 Sample density versus recovery depth for Siqueiros samples.................................171

7-20 Sample MgO content versus recovery depth for Siqueiros samples......................171

7-21 Density of samples vs. depth with addition of olivine phenocrysts.......................172

7-22 Density vs. depth of Siqueiros samples with 5 modal % olivine added to picritic
and olivine rich basalts. .................. ................................................................ ....... 172

7-23 M agma transport within the Siqueiros transform................................. ............ 179

A-1 Cameca microprobe data versus ARL and JEOL microprobe data...........183

A-2 Normalized Cameca microprobe data. ..........................................184

F-i Linear regression of Na20................................................................ ......... .............239

F-2 Linear regression of FeO ......... ........ ......... ....................239

F-3 Polynom ial regression of N a20 ....................................................... 240

F-4 Polynomial regression of FeO............................................ .... ... .....240
















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

INTRA-TRANSFORM VOLCANISM ALONG THE SIQUEIROS FRACTURE ZONE
8020' N 8030' N, EAST PACIFIC RISE

By

Michelle Renae Hays

December 2004

Chair: Michael Perfit
Mojor Department: Geological Sciences

Detailed sampling and sonar mapping of the Siqueiros transform were completed in

1991 during the Atlantis-II 125-25 Research Cruise. Fresh, glassy, volcanic rocks were

recovered from small constructional volcanic landforms within leaky transform faults and

from troughs within the transform. Three of the troughs within the transform exhibit

organized spreading and are believed to be intra-transform spreading centers that have

resulted from changes in the relative motions of the Pacific and Cocos plates. The

samples recovered include extremely primitive lavas (pricritic and olivine-phyric basalts

to high-MgO basalts). Compared to the adjacent 9-100N segment of the EPR the

Siqueiros basalts are more primitive and tend to group on the more depleted end in major

and trace element diagrams. Four chemically distinct groups of lavas have been

identified within the transform. The spreading centers have erupted only N-MORB type

lavas which are similar to those from the EPR. Lavas recovered from shear zones within

the transform tend to be more primitive and depleted in incompatible elements with the









most incompatible element depleted lavas (D-MORB) recovered from the A-B fault, the

shear zone connecting the two western most spreading centers. The E-MORB samples

were only recovered at the western ridge-transform intersection (WRTI) and a group of

low Na20 samples were recovered within spreading center A. Fractional crystallization

models indicate that the majority of the N-MORB samples can be explained by 50-60%

fractional crystallization of olivine spinel + plagioclase + clinopyroxene of 2-3 parental

compositions similar to the high-MgO lavas recovered from the A-B fault. Scatter about

CaO vs. A1203 trends and ratios among highly incompatible elements, along with

variations in phenocrysts compositions, indicate that mixing between primitive and

evolved compositions is needed in order to explain the entire range of major and trace

element variations. Resorbtion textures and chemical analysis of many the large

phenocrysts show they are out of equilibrium with the host magma and were derived

from high CaO, high MgO lavas. REE diagrams show that the D-MORB samples from

the A-B fault cannot be related to the N-MORB samples by fractional crystallization

alone. Mixing models indicate that N-MORB compositions can be produced by mixing

of approximately 4-6% of an E-MORB composition with the D-MORB samples. The

low Na20 samples from spreading center A are best explained by mixing with a more

depleted source, but Na8.o and Fe8.o data indicate both mixing of sources and variable

extents and depths of melting occur within the transform. The compositional variations

of the Siqueiros samples can be explained by a petrogenetic model in which lava

compositions are controlled by the presence/absence, size, and depth of melt lenses

within the transform.














CHAPTER 1
INTRODUCTION

The Siqueiros transform fault is a left lateral transform fault located on the

Northern East Pacific Rise (NEPR) between 8020'N and 8030'N (Figure 1-1). The

transform domain is approximately 20 km wide and offsets the NEPR by 138 km (Fornari

et al., 1989). It lies along a fast-spreading portion of the EPR with a half-slip rate of

approximately 63 km Ma-1 (Fomari et al., 1989). In 1991, detailed observational data and

extensive sampling revealed 3 intra-transform spreading centers and small eruptive

centers within the transform shear zones, all of which exhibit recent volcanism (Perfit et

al., 1996).

Transforms faults, such as Siqueiros, theoretically parallel the direction of plate

motion and are conservative plate boundaries where no plate construction or destruction

is thought to occur. Volcanism within the Siqueiros transform is unusual, but is believed

to be the result of counterclockwise changes in the spreading direction between the

Pacific and Cocos plates. Rotations in plate motions resulted in an extensional

environment within the transform (Pockalny et al., 1997). Petrologic and morphologic

data suggest that volcanism does occur within other transform domains that exhibit

extension, especially along fast- and superfast-spreading portions of the Mid-Ocean

Ridge crest (Perfit et al., 1996; Fomari et al., 1989). Few of these transforms have been

sampled or studied in any great detail (Hekinian et al, 1995; Wendt et al., 1999). A few

samples have been analyzed from the Raitt transform along the Pacific-Antarctic

spreading ridge (Castillo et al., 1988) and some samples have been recovered from the











Blanco transform between the Juan de Fuca and Gorda Ridges (Embley & Wilson, 1992;


Tierney, 2003). The Garrett transform on the southern EPR is the only oceanic transform


where magmatism has been extensively studied and samples have been analyzed. The


processes that formed the lavas erupted in these environments and their relations to the


nearby ridges are still poorly understood.





20'N-
Rivera
-105 30' -105 00 -104 30 -104' 00' 5N- Pac
.I Plate O Cocos

Pacific
Ocean -
I /~.Squeiro
10 ""' 110"N I05"N -10 N





,*,r *"



























Di1h rrm
Figure 1 -1. Location map of Siqueiros transform.









Because of their presumed colder thermal environment, intra-transform lavas are

removed from the large magma chambers beneath the ridge, in which large volumes of

melt are mixed. The study of volcanic transforms may provide new insights into the

scale of mantle heterogeneities and the compositions of the depleted and enriched mantle

because such components may not be thoroughly mixed in areas removed from the larger

magma chambers beneath ridge segments. The Siqueiros transform offers a unique look

at three, small, focused spreading centers which are separate from the large magma

chambers beneath the EPR, which are believed to be sites where different mantle

components are mixed. The Siqueiros transform also contains samples of primitive

compositions rarely found elsewhere in close proximity to evolved samples.

The goal of this study is to gain a better understanding of the petrologic

segmentation and magmatic processes beneath the Siqueiros transform and to compare

the basalts of the Siqueiros transform domain with those of the adjacent EPR and Garrett

transform. Major and trace element variations in concert with crystallization and mixing

models have been used to estimate parental magmatic compositions. Phase chemical data

and incompatible element ratios and variations have also been used to evaluate the

histories of crystallization and mixing.

Sampling of the Siqueiros Transform

The study of fracture zones is important because rocks that are believed to compose

the lower oceanic crust and upper mantle (gabbroic and ultramafic rocks) are commonly

found within fracture zones and rarely found elsewhere in the ocean basins. The

Siqueiros transform was originally investigated to complement the knowledge gained

from the slow moving Fracture Zone A in the FAMOUS area of the Mid-Atlantic Ridge

(Detrick et al., 1973; Crane, 1976). The first near bottom geological and geophysical









survey was conducted at the western intersection of the Siqueiros transform fault and the

East Pacific Rise (EPR) using a Deep Tow Fish (Crane, 1976). Samples were also

recovered from the western most Siqueiros transform and adjacent EPR by rock dredging

(Crane, 1976; Batiza et al., 1977; Natland, 1989). Sampling revealed a broad range of

rock types, which include enriched mid-ocean ridge basalts (E-MORBS), normal mid-

ocean ridge basalts (N-MORBS), and high-MgO rocks, but a lack of precise locations for

the dredged samples made it difficult to interpret the geochemical data in this area of

complex sea-floor structure (Natland, 1989). The acquisition of a Sea MARC II sonar

survey in July 1987 (Fornari et al., 1989) and Alvin submersible dive observations in

May-June 1991 (Fornari et al., 1991) has allowed a better understanding of the seafloor

structure. The petrologic, observational, and morphologic data from the 1987 and 1991

cruises revealed what appeared to be sites of intra-transform spreading (Fornari et al.,

1989; Fornari et al., 1991; Perfit et al., 1996). Four troughs labeled A, B, C and, D and

five strike-slip faults were identified using the bathymetric and side-looking sonar data

(Fornari et al., 1989). During Alvin submersible dives, fresh-looking pillow lava flows

and sheet flows were identified along the spreading ridges and eruptive centers were

found in transform shear zones (Figure 1-2) (Fornari et al., 1991). Despite their location

far from the north and south tip of the East Pacific Rise, the basalts recovered from three

of the troughs (A, B, and C) still have relatively unaltered glassy rinds, which suggests

that they have been recently erupted and do not originate from the spreading associated

with the adjacent East Pacific Rise. The forth trough, D, was found to comprise an older

volcanic terrain, which has been strongly tectonized. Trough D does not contain any

identifiable ridges and any spreading is believed to be focused along transform parallel












-104


j ~ i Sample Locations
T*- .0- a A Spreading Center A
*' ;! t "SW Fw~i f a.,~ i~" A-B Fault
A A Spreading Center B
i g *r i 0 B-C Fault
r' j IA Spreading Center C




















Figure 1-2. Sample location map.
go C-D Fault
r iTrough D
AN Ie W-RTI





Map Location







8 08


-104 -103
Figure 1-2. Sample location map.


-103









lineaments (Fornari et al., 1991). Troughs A, B, and C are believed to be small intra-

transform spreading centers (Fomari et al., 1991) where organized spreading is occurring.

The lavas are remarkably fresh with little sediment cover and thick glassy rinds. Small

constructional volcanic landforms were found at small offsets within the strike-slip faults

connecting the spreading centers. The samples within the A-B fault were unusually

mafic, olivine-rich basalts (Perfit et al., 1996). The olivine-phyric basalts are referred to

as picritic basalts. In 1996, Perfit and others conducted a detailed study of the young

picritic basalts and high-MgO lavas from the A-B fault. The picritic basalts were found

to be formed by the accumulation of olivine and minor spinel from high-MgO melts

(Perfit et al., 1996). The high-MgO glasses recovered from the strike-slip fault were

found to potentially be near-primary melts from incompatible-element depleted oceanic

mantle that have been little modified by crustal mixing and or fractionation processes

(Perfit et al., 1996). The Siqueiros samples collected in 1991 are petrologically diverse

and contain picritic basalts, ferrobasalts, FeTi basalts, N-MORB, incompatible element

depleted normal mid-ocean ridge basalts (D-MORB) and E-MORB. This study will

combine the previous work completed on the picritic basalts from the A-B fault with a

more detailed examination of the samples recovered from other localities within the

transform domain in order to gain a better understanding of the petrologic and tectonic

evolution of the Siqueiros transform.

The Transform Fault Effect

Geophysical studies indicate that an axial magma chamber overlain by a thin melt

lens is present beneath many sections of the EPR (Sinton & Detrick, 1992; Dunn et al.,

2000). The seismic reflector representing the melt lens is <3 to 4 km wide and caps a low

compressional wave velocity zone 5-7 km wide. This low velocity zone is believed to









represent the presence of melt mixed with crystals to produce a "mush". Near 9030'N on

the EPR the axial magma chamber reflector was found to be 1-2 km below the rise.

Rosendahl et al. (1976) and Orcutt et al. (1976) demonstrated the existence of a crustal

low-velocity zone about 4 km wide and 0.5 to 1.0 km beneath the crest of the EPR in the

vicinity of the Siqueiros Fracture Zone. Although seismic studies have not been

conducted beneath the Siqueiros fracture zone, seismic studies have shown that the axial

magma chamber seismic reflector terminates near fracture zones and is reduced at other

discontinuities (Macdonald and Fox, 1988; Macdonald et al., 1991). The lack of a large

magma chamber beneath transforms allows for the possible eruption of unmixed mantle

components.

Studies have shown that basalts adjacent to fractures zones tend to be

characterized by more fractionated compositions (Melson and Thompson, 1971; Hekinian

and Thompson, 1976; Natland and Melson, 1980; Christie and Sinton, 1981; LeRoex and

Dick, 1981, Sinton et al., 1983; Fomari et al., 1983; Perfit and Fornari, 1983; Perfit et

al.,1983; Langmuir and Bender, 1984; Elthon, 1988). At the Siqueiros ridge transform

intersection (RTI), highly fractionated E-MORBs along with a few FeTi basalts have

been recovered. A wide range of magma compositions including highly-fractionated

magmas have been found proximal to fracture zones (Elthon, 1988). These observations

have been explained by the "transform fault effect" in which the isotherms are suppressed

near the transform due to the juxtaposition of older, cooler lithosphere against the ridge

transform intersection (Langmuir & Bender, 1984). The depressed isotherms may allow

isolated magma pockets near the ridge transform intersection permitting the highly

fractionated magmas to be developed (Perfit and Fornari, 1983).










Mantle Heterogenities

Initial studies along mid-ocean ridges found axial lavas to be rather homogeneous,

but more intensive studies along strike have revealed variations in basalt chemistry that

are believed to relate to ridge segmentation and morphology (Thompson et al., 1985;

Langmuir and Bender, 1986; Smith et al., 1994; Bazin et al., 2001). Transform faults are

first order segments that partition the ridge into distinctive tectonic units which persist for

a million years or more and have been found to separate ridge segments with contrasting

tectonic and petrological properties (Macdonald et al., 1988). Smaller second and third

order segments, such as overlapping spreading centers, deviations in axial linearity

(DEVALS), small non-overlapping offsets (SNOOs), and kinks in the ridges, have also

been found to correlate with geochemical segmentation (Langmuir and Bender, 1986;

Bazin et al., 2001; Smith et al., 2001). The axial discontinuities can be related to the

axial magma chamber's depth beneath the seafloor, width, thickness, continuity along the

ridge, and the geochemistry of the erupted lavas (Macdonald, 1998). The axial

discontinuities have also been found to be related to the volcanic segmentation of the

ridge (White et al., 2002). Lava morphology (from sheet to pillow flows) has been found

to coincide with boundaries of morphologically defined third-order tectonic segments of

the ridge crest and to indicate reduced eruption rates (White et al., 2002).

Studies of mantle heterogeneities have recently focused on across strike sampling

in order to study the chemistry of off-axis eruptions, which may tap different sources

without mixing in large magma chambers or mush zones. Studies on the East Pacific

Rise (ERP) that have focused on across strike variations have found some off-axis lavas

that appear to be younger than the surrounding terrain and show greater chemical

variability than axial lavas. Remarkably small-scale spatial variations in basalt chemistry









of these off-axis lavas have been found (Reynolds et al., 1992; Perfit et al., 1994; Bideau

and Hekinian, 1995; Perfit and Chadwick, 1998; Castillo et al., 2000). Detailed off-axis

studies have revealed the existence of lavas with distinctive chemical compositions, both

more and less enriched in incompatible elements than those delivered to the axis. Some

are similar to depleted lavas recovered from near axis seamounts (Fornari et al., 1988;

Perfit et al., 1994; Reynolds and Langmuir, 2000). A nonsystematic distribution ofE-

MORB lavas was found off-axis in the 9-10' N region of the NEPR (Perfit et al., 1994,

Perfit and Chadwick, 1998; Smith et al., 2001). It is believed that these resulted from

frequent low-volume off-axis eruptions that did not reflect mixing within the large

magma chamber beneath the ridge. Significant chemical variation at 9031 'N was found

to be on the scale of 200 m and is believed to result from both rapid changes in magma

chamber chemistry and frequent low-volume on-axis and off-axis eruptions (Perfit et al.,

1994). Off-axis flows have been documented up to 4 km from the ridge axis along the

EPR (Goldstein et al., 1994; Perfit and Chadwick, 1998; Schouten et al., 1999; Reynolds

and Langmuir, 2000). The source of these magmas is poorly understood. They may be

fed by axial eruptions that flow great distances off-axis or they may be associated with an

off-axis magma chamber. Transforms are also removed from the well-mixed large

magma chambers associated with the ridge axis. The intra-transform spreading centers

can help provide a better understanding of the scale and composition of mantle

heterogeneities and may be important in understanding the source of off-axial eruptions.














CHAPTER 2
GEOLOGICAL RELATIONSHIPS AND SAMPLE LOCALITIES IN THE
SIQUEIROS TRANSFORM

The Siqueiros transform is comprised of a number of different sections that are

morphologically distinct (Figure 2-1). The entire transform domain is about 20 km wide

and includes the transform valley and adjacent seafloor that has been morphologically or

structurally affected by proximity to the transform. Within the transform domain exists

the transform tectonized zone (TTZ) and the transform fault zone (TFZ). The TTZ is

defined as the area truncated by abyssal hill topography on opposite sides of the

transform valley. The TFZ is usually a 2 km wide continuous swath of lineated ridges,

troughs, and closed contoured basins. The Siqueiros transform domain has been found to

contain both shear and spreading related features. The shear related features are a series

of 5 en echelon TFZ which consist of ridges and troughs that nearly parallel the Pacific-

Cocos relative plate motions. The TFZ are approximately 15-25 km long and are the

focus of recent strike-slip deformation. The fault troughs are deep (up to 3650 m) and

narrow (1-3 km wide) especially in the western portion of the transform (Fornari et al.,

1989). The spreading related features consist of four extensional relay zones (ERZ),

which are equivalent to continental pull-apart basins. These ERZ are believed to have

resulted from a series of counterclockwise changes in spreading direction that occurred at

about 3.5, 2.5, 1.5, and 0.5 Ma (Pockalny et al., 1997). Very fresh-looking lava flows

and systematic aging of the seafloor across the axis and flanks were found at 3 of the

relay zones during Alvin submersible dives confirming that there are three spreading






























104030' 104000'W 103030'W 103000"W


Figure 2-1. Plate boundary geometry of the Siqueiros transform. Locations of the 3 troughs that exhibit intra-transform spreading (A,
B, and C) and the fourth trough (D), which has been strongly tectonized and does not exhibit organized spreading are
shown. Dashed lines show TFZ (A-B fault, B-C fault, C-D fault) and the two faults that connect the spreading centers to
the RTIs (WRTI-A and ERTI-D). Light shaded box depicts the transform domain. Darker shaded boxes represent the
WRTI and ERTI. Adjusted from Fornari et al., 1989.










centers (A, B, and C) within the transform. The fourth pull-apart basin (trough D) was

strongly tectonized and did not exhibit any organized spreading. The intra-transform

spreading centers may have begun as leaky transforms that evolved into small well-

developed spreading centers with the persistent change in the plate geometry (Pockalny et

al., 1997). The geologic locations referred to in this study include the western ridge

transform intersection (WRTI), the eastern ridge transform intersection (ERTI), the 3

spreading centers (A, B, and C), trough D, and the TFZ. For this study, the transform

faults were divided into offsets between the three spreading centers (A-B fault, B-C fault,

and C-D fault). The A-WRTI and D-ERTI fault offsets were included as part of the

WRTI and ERTI, respectively.

At the ridge transform intersections (RTIs) the northern and southern limbs of the

EPR axis become slightly deeper and swing into the transform domain, which is

morphologically characteristic of transforms at the fast-end of the slip rate spectrum.

Both the eastern ridge transform intersection (ERTI) and the western ridge transform

intersection (WRTI) have unrifted crest and have abyssal hill topography characteristic of

fast to medium spreading ridge segments.

Spreading centers A and B both exhibit bilateral symmetry about the spreading

axis out to 20-40 km. Spreading center A is the western most trough and is connected

with the WRTI by a TFZ. It is a sigmoid-shaped basin that consists of two ridges

(Fornari et al., 1991). Lavas on the northern ridge are older and heavily overprinted. The

southern portion has younger pillow lavas, but they are overprinted with faults and

fissures, suggesting that transform tectonics are influencing the area (Fornari et al., 1991).

East of the southern A axial ridge the volcanic terrain is older and extensively weathered.









West of the spreading center the transform fault intersects the EPR and lavas age to the

north as the EPR is approached. Spreading center B is the most well developed spreading

center with abyssal hill structures up to 8 km long. The youngest looking flows were

found on a small 100 m cone near the central portion of the axis. Spreading center B also

consists of pillow walls and constructional pillow escarpments.

Troughs C and D have much smaller swaths of abyssal hill topography (10-20 km).

Spreading fabric could only be identified within trough C. Fresh volcanics were found

only within the floor of the graben and along the walls of the graben. Many of the flows

within C are sheet flows. Trough D was found to contain only strongly tectonized older

volcanic terrain. Fresh basalts were recovered north of D suggesting that any spreading

at D is focused along a transform-parallel lineament (Fornari et al., 1991).

Transform faults A-B and B-C were chosen for detailed studies using ALVIN and

the rock dredge because they link the most morphologically distinct and best organized

intra-transform spreading centers. The fault zones are also very clear and have relief

between 1000-1500m. Also, the axial deeps and RTI deeps are well-defined. The faults

are approximately parallel (078' [A-B] and 075' [B-C]) to the relative plate motion of the

Pacific and Cocos plate (0820). Within the A-B transform young glassy picritic basalts

and olivine-phyric basalts were collected by dredging and ALVIN sampling. The young

volcanic centers were found along the lower parts of the south and north walls of the

transform adjacent to and overlying much older highly-sedimented terrain of talus,

pelagic sediment, and older manganese encrusted basalt. There is no indication of recent

faulting within the young volcanics. The B-C fault is much shorter than the A-B fault (35

km vs. 18 km) and has less relief. N-MORB samples were primarily recovered from









within this transform. The C-D fault and the fault linking trough D and the ERTI are less

well-defined and the linearity of the faults are not continuous to the SEPR.

Samples used for this study were collected in 1991 aboard the Atlantis II cruise

125-25 and include rock dredges, rock cores, Alvin submersible dives (Figures 2-2 and 2-

3). Eleven SeaBeam surveys were also conducted during the 1991 cruise to add to prior

SeaBeam and Sea MARC II survey data. The SeaBeam data allows identification of

morphological features that have 10 m or more relief. Seventeen Alvin dives were

completed within the Siqueiros transform domain and 171 samples were collected (Table

2-1). Sample localities and geological relationships are based on the ALVIN dive

observations and SeaBeam survey data. Alvin dive tracks are based on the ALNAV

network and SeaBeam maps with an estimated uncertainty of 100-200 m. Thirty-nine

dredges and five rock cores were also conducted in the Siqueiros transform domain

(Table 2-2). The dredges consisted of a 50 cm x 1 m mouth frame, 2 m chain bag with

fishnet liner, and chain harness, with a 12,000 lb weaklink system. A cylindrical, lead

depressor weight was used 100 m up the wire from the dredge mouth. Poor performance

of onboard pingers forced the wire to be between 300-400 m greater than bottom depth to

insure contact with the bottom. Dredge tracks were kept short (<1 km) to maximize

confidence in the sample localities and were located using Global Positioning System

(GPS) and by correlating real-time Seabeam depths to existing maps along the sample

track.

The Siqueiros volcanic terrain mainly consists of pillow flows found within and

around the intra-transform spreading centers and at small eruptive centers in transform

shear zones. A few sheet flows have been found within the spreading basins. In contrast,









Table 2-1. Siqueiros transform Alvin dive locations.
# of General Sample
Dive General Location of Dive S ip Smn
Samples Descriptions
2375 2nd abyssal hill west of spreading 9 Fresh pillow basalts, one
center B axis sediment sample
2376 Southern portion of spreading center 11 Fresh to slightly weathered
B axis pillow basalts, one ropy
lava
2377 Northern portion of spreading center 11 Fresh basalts
B axis
2378 Southern crescent ridge of C and 11 Fresh pillow and sheet
central graben basalts
2379 North wall of A-B fault just west of 3 Sediment covered micro-
spreading center B intersection gabbros
2380 Southern RTI hole at spreading 12 Older pillow, lobate, and
center B and trough east of spreading sheet basalts
center B axis
2381 Southern wall of B-C fault 13 Older, somewhat
weathered basalts
2382 Southern wall west of spreading 11 Fresh older sediment
center B and plateau south of covered basalts. Some
transform lobates and sheets.
2383 Southern ridge of spreading center A 8 Very fresh pillow basalts to
slightly weathered basalts
2384 Young cones in axis of A-B fault 14 Very fresh, glassy basalts
to older basalt fragments
2385 Northern RTI hole and central rift of 9 Sheet, lobate, and pillow
spreading center C axis basalts, fresh to slightly
weathered
2386 Trough and northern peak of D 8 Somewhat young hackly
lava, mostly Mn coated
older pillow and lobate
basalts
2387 Cones in axis of B-C fault near 9 Fresh-slightly altered
intersection with spreading center C basalts, mostly pillows
2388 A-B fault, cone on south side of axis 14 Older sediment covered
and traverse up the north wall basalts and microgabbros
2389 Northern ridge in spreading center A 8 Fresh pillow lavas
2390 WRTI, small ridge that connects EPR 9 Fresh sediment covered
to south wall of the A-B fault pillow and lobate basalts
2391 Small cone built against south wall of 11 Sediment covered basalt,
the A-B fault west of southern microgabbros
spreading center A ridge









Table 2-2. Siqueiros transform dredge locations.
Dredge/Rock Core General Location of Dredge/Rock Core # of Samples
A25-D1 Southern portion of spreading center B 48
A25-D2 B-C Fault 24
A25-D4 West of spreading center B 12
A25-D5 West of spreading center B 5
A25-D6 West of spreading center B 2
A25-D7 West of spreading center B 3
D25-D8 West of spreading center B 1
A25-D9 West of spreading center B No Recovery
A25-D10 West of spreading center B No Recovery
A25-D12 South of spreading center B No Recovery
A25-D13 South of spreading center B 1
A25-D14 South of spreading center B 6
A25-D15 South of spreading center B 1
A25-D16 Northwest of A-B fault No Recovery
A25-D17 Northwest of A-B fault 10
A25-D18 Small ridge parallel hill west of spreading center B 6
A25-D19 South end of spreading center B 10
A25-D20 Small cones near the midpoint of A-B fault 12
A25-D22 A-B fault 5
A25-D23 A-B fault 2
A25-D24 A-B fault 3
A25-D25 Southwest side of spreading center C 7
A25-D26 North of spreading center C 10
A25-D27 West of spreading center C 5
A25-D28 Eastern Ridge Transform Intersection 5
A25-D29 Eastern Ridge Transform Intersection 1
A25-D30 Eastern Ridge Transform Intersection 8
A25-D31 Eastern Ridge Transform Intersection 1
A25-D32 Spreading center C 6
A25-D33 Spreading center C 5
A25-D34 Spreading center C 5
A25-D35 Southwest of spreading center A 7
A25-D36 East of spreading center A 9
A25-D37 East of spreading center A 3
A25-D38 East of spreading center A 5
A25-D39 EPR abyssal hills 4
A25-D43 Eastern Ridge Transform Intersection 5
A25-D44 Eastern Ridge Transform Intersection 2
RC-3 North of spreading center B Little
RC-11 South of spreading center B Little
RC-40 Eastern Ridge Transform Intersection 1
RC-41 Eastern Ridge Transform Intersection 1
RC-42 Eastern Ridge Transform Intersection 1








17


the EPR north of the transform consists mainly of sheet or lobate flows emanating from

the axis and occasional pillow flows, which are found near ridge tips and off axis (Perfit

and Chadwick, 1998). Pillow flows are characteristic of low effusion rates suggesting

that the intra-transform spreading centers are not as magmatically active as the adjacent

EPR segments. The northern segment of the EPR extending up to the Clipperton

transform is very well sampled. Little sampling has been done on the southern limb of

the EPR.














103045'


A Spreading Center A
* A-B Fault
* Spreading Center B


104000'
B-C Fault
A Spreading Center C
F C-D Fault


103045'
O Trough D

* Ridge Transform Intersection
/\/100 m Contours


Figure 2-2. Bathymetry and sample locations for west side of the Siqueiros transform.


103030'


104000'


10330'














103015'


A Spreading Center A
A-B Fault
* Spreading Center B


103030'
4 B-C Fault
A Spreading Center C
D C-D Fault


103015'
Q Trough D

0 Ridge Transform Intersection
/N/100 m Contours


Figure 2-3. Bathymetry and sample locations for east side of the Siqueiros transform.


103000'


103030'


103000'
















CHAPTER 3
ANALYTICAL METHODS

All Alvin samples were petrographically described and cataloged on ship.

Representative dredge samples were inspected and slabbed with a rock saw for thin-

section chips and in order to remove surface alteration. Glass rinds were removed from

samples with glass and separated for further cleaning. The glass and some whole rocks

were crushed in a hardened steel mortar and then cleaned in acetone, 2N HC1, and

distilled water in a heated ultrasonic bath. The samples were then inspected under a

binocular microscope and any alteration, sediment, or Mn-encrusted glasses were

removed. A few samples that were heavily Mn-encrusted could not be completely

cleaned and were labeled as "dirty" samples. After cleaning, 7-10 grams of glass or rock

chips were crushed and powdered. The remainder of the clean samples was saved for

other analyses. Over 150 samples were processed at sea.

Major element analysis of the Siqueiros natural glass samples was done by

electron microprobe at the US Geological Survey (USGS) in Denver using an ARL-

SEMQ microprobe and JEOL microprobe. An additional data set for major and minor

elements was produced for the Siqueiros samples by analysis on a Cameca SX50 electron

microprobe at the University of Tasmania (Danyushevsky, personal comm.). For the

electron microprobe analysis, sub-samples of the cleaned glass chips were inspected

using a binocular microscope and selected for analysis. All probe analyses were

normalized to standard glasses VG-A99 and JdF-D2 which were run concurrently with









the Siqueiros glasses. Analyses were corrected using the procedure of Meeker and Quick

(1991). Whole-rock samples (glass plus phenocrysts) were also analyzed by microprobe

at the USGS after fusion in a rhenium strip furnace.

Most of the Siqueiros samples were analyzed for trace element contents (Co, Cu,

Ga, Nb, Ni, Rb, Sr, Y, Zn, Zr, V, Cr, Ba, Sc, K, and Ti) by x-ray fluorescence

spectrometery (XRF) in the department of Geological Sciences at the University of

Florida using an automated ARL-8420+ spectrometer. Approximately five grams of the

powder samples were mixed with an organic binder and pressed into pellets for XRF

analysis. Matrix absorption effects were accounted for based on the intensity of the Rh

Compton peak (Reynolds, 1963). Standards were run no less frequently than every seven

samples in order to correct for any fluctuations in the x-ray intensity or instrument

conditions. Replicate analyses of rock standards show that accuracy and precision are

generally better than 2% for the elements Cu, Zn, V, Ti, Sr, Y, and Zr, better than 5%

for K, Rb, Nb, Ba, Co, Ga, and Ni, and to within 10 % for Sc. Analytical precision is

significantly worse (> 20%) for Nb, Rb, and Ba when abundances are near detection

limits (3, 2, 10 ppm respectively). Direct current plasma (DCP) spectrometry was also

used for phenocryst free glass separates at the Lamont-Doherty Earth Observatory. The

DCP analysis included major and some minor and trace elements (Ba, Cr, Cu, Ni, Sc, Sr,

V, Y, Zr, Mn, and Ti). A few samples were measured for trace elements (Y and Sc) and

the rare earth elements (REEs) by inductively coupled plasma- mass spectrometry (ICP-

MS) at the University of Houston by Dr. John Casey. Major element and trace element

data was also analyzed by the Canadian Geological Survey by ICP-ASE. Chemical









analyses of the Siqueiros picritic basalts were done by microprobe after fusion in a

rhenium strip furnace.

Duplicate analyses of selected samples were completed by the different labs and by

different methods. Because this has the potential to lead to systematic bias in the data, all

of the data were graphically compared (Figure 3-1). The microprobe data from the ARL-

SEMQ microprobe and the JEOL microprobe show no apparent analytical offsets. Nor

are there any significant differences between the microprobe data and the DCP data.

However, the MgO, and P205, data from the Cameca SX50 electron microprobe appear to

have a slight offsets when compared to the other Siqueiros electron microprobe data. The

Cameca SX50 electron microprobe MgO and P205 data was normalized to match the

other electron microprobe MgO and P20s data (Figure 3-2). The normalization method is

discussed in Appendix A. The Cr203 contents obtained on the Cameca SX50 electron

microprobe are not directly comparable to most of the other Cr data (mostly XRF).

Measurements are made on the glass composition whereas the XRF analyses are of the

whole rock samples. Therefore, it was not possible to use the microprobe Cr203 contents

to compare with the Cr203 contents of the other Siqueiros samples. Since the XRF is a

much more accurate method for obtaining Cr contents, only the Cr203 contents

determined by XRF were used in this study.



















ARL-SEMQ Microprobe Samples
JEOL Microprobe Samples
E Cameca SX50 Samples
+ DCP Samples
A A


L +
/4^



-E]
'"'I-- A




L L


A A


*" CMl. A
E -P4
L k111k LL L
L A.L L

A *^

ALL"


EL


& z


&L
L k -


5 6 7 8 9
MgO


10 11 12 5 6 7 8 9
MgO


10 11 12


Figure 3-1. Graphical comparison of ARL microprobe, JEOL microprobe, DCP, and Cameca SX50 data before correction of the data.


2
0

1.5


E*


-L L

L
z-.
z h


z
2.6 "O


L

k L hk


4k
z,4E


- 12.5

-12


11.5
0
11 M

10.5




















ARL-SEMQ Microprobe Samples
JEOL Microprobe Samples
o Cameca SX50 Samples
+ DCP Samples


Iv




LA


6 7 8 9 10
MgO
T kf
.A q1 ^


11 12


kt Ai kk-
L- L
A >
_- ** ^,A;^^,


h:~Lt
A\L


L
b~L -


5 6 7 8 9 10 11 12
MgO


Figure 3-1. Continued.


0.3

0

0.2



0.1





0.7

0.6

0.5

0.4 -

0.3


', i


r ir I






















ARL-SEMQ Microprobe Samples
JEOL Microprobe Samples
o Cameca SX50 Samples
+ DCP Samples


I
a ..
-

A '


k kk
t


ll


5 6 7 8 9 10 11 12 5 6 7 8 9 10 11 12


MgO


MgO


Figure 3-2. Comparison of data after adjustment of the Cameca SX50 MgO and P205 contents.


2.5


IA I
2.' ~-

I
& k

'' s,, ~ ^t


z
2.6 n


hh
A _k


AA A-


L L
kbp


4 4

h. s^





~ Y~


















A ARL-SEMQ Microprobe Samples
JEOL Microprobe Samples
E Cameca SX50 Samples
+ DCP Samples


LA +


J _
I IA li I


5 6 7 8 9 10 11 12 5 6 7 8 9 10 11 12


MgO


Figure 3-2. Continued.


St
At


'1%


-0
0.3 "0


0.2


0.7

0.6

0.5

0.4
0
0.3

0.2

0.1


MgO









The chemical characteristics of the samples were determined by comparing their

major and trace element abundances to each other and samples from the adjacent East

Pacific Rise (EPR) and from the Garrett transform. Liquid lines of descent (LLDs) and

rare-earth element diagrams were used to help group samples of similar parental

compositions. SeaBeam and SeaMarcII sonar data has also been collected for the

Siqueiros transform (Fomari et al., 1989). The sonar data along with the ALVIN

submersible dive observations were used to determine sample locations with respect to

the local geologic/structural features. The ALVIN submersible observations and dive

track data were used to precisely locate samples and to create depth profiles along dive

transects. GIS (Arcview) data files were created that consist of latitude/longitude,

elevation (depth), geologic location, and chemical characteristics. The data files were

then used to create geologic maps of the Siqueiros transform.

Thin-sections of 60 samples were studied with a petrographic microscope to

identify the different phases in each sample and to provide information regarding

crystallization and mixing histories during petrogenesis. Microprobe analyses of spinel,

olivine, and plagioclase phenocrysts were also completed for many of the samples. The

compositions of the phenocrysts were then used to better understand crystallization and

mixing histories.















CHAPTER 4
PETROGRAPHY AND LOCAL GEOLOGIC RELATIONSHIPS

The rocks from the Siqueiros transform include picrites, picritic basalts, basalts,

and a few microgabbros. Thin sections examined in this study were cut from the outer

glassy rinds as well as the more crystalline interiors of 63 samples. The majority of the

samples chosen for thin section analysis were recovered from the A-B fault, but thin

sections were made from samples from all spreading centers and faults and of one sample

from the RTI. Descriptions of the thin sections examined are provided in Table 4-1 and

representative photomicrographs are shown in Figures 4-1, 4-2, and 4-3. A few of the

samples are aphyric or vitrophyric, containing less than 1 volume % phenocrysts and

microphenocrysts, but most of the samples are phyric containing greater than 5%

microphenocrysts.

Samples from the A-B Fault

The majority of the thin sections are from the A-B fault because these samples are

unusually olivine-rich with 5-20 modal% olivine phenocrysts (Perfit et al., 1996). The

samples are remarkably fresh and unaltered, with thick glassy rinds which are free of

palagonitization and Mn-coatings; indications of the relative youthfulness of the lava

(Perfit et al., 1996). The samples differ from the rest of the Siqueiros samples in that they

contain only olivine and spinel phenocrysts. Abundant olivine microphenocrysts are

found in the glass and at centers of variolites. Near the interiors there is minor dendritic

plagioclase microphenocrysts radiating from olivines. Despite the great recovery depths

(3000-3900m), many of the olivine-rich basalts from the A-B fault have a greater degree














Table 4-1. Thin section descriptions.
Sample MgO glass Ve phenocryst texture microphenocryst texture
Loc glass Ve remarks
# wt% olivine plag spinel olivine plag cpx
Swallow Porphyritic; variolitic; flow
Rounded, Hopper- tail features around
2389-3 A ND Y Y embayed Embayed, zoned None euhedral tabular None phenocrysts; few plag clots
Slight zoning, Swallow
Very few, embayed, tail, Porphyritic; plag. clots;
2389-8 NA D Y Y rounded skeletal None Subhedral acicular -None variolitic intersertial
Rounded, Huge, embayed, Acicular- Porphyritic; huge plag. clots
2389-8A A ND Y Y skeletal skeletal, zoning None Subhedral tabular None up to 9 mm; variolitic
Oscillatory
zoning, skeletal, Porphyritic; variolites around
2389-1P A 7.35Y__ Few Rounded rounded None Variolitic Variolitic None ol microphenocrysts
Up to 6 mm,
rounded, Skeletal, rounded, Porphyritic; primarily
skeletal, inside & outside Hopper- Swallow microphenocrysts;
2384-1 A-B 9.6 Y Y embayed None olivine dendritic tail None intersertial with opaques
1-4 mm, Inside & outside Swallow Porphyritic; variolitic -
2384-10 A-B 9.59Y__ Few skeletal None olivine Dendritic tail None dendritic; large ol. clots
Porphyritic; variolitic-
intersertial; primarily
Very Few, Few inside olivine Hopper- Swallow quenched glass with plag. &
2384-11 A-B 8.79Y few embayed None microphenocrysts subhedral tail None ol. microphenocrysts
Swallow
Very Skeletal, Very few, Rounded; inside & tail Porphyritic; ol. & plag. clots;
2384-12 A-B 9.11 few rounded skeletal outside olivine Dendritic tabular None intersertial with opaques
Green alteration in vesicals;
variolitic-dendritic with
1-2 mm, Swallow opaques; dendritic growth
2384-13 A-B 8.53Y_ Y skeletal None Inside olivines Dendritic tail None around ol.
6-7 mm, Swallow Porphyritic; variolitic-
rounded, Skeletal, zoned tail dendritic with opaques;
2384-3 A-B 10.1 Few skeletal None edges Hopper dendritic None primarily microlites













Table 4-1. Continued.
Sample MgO glass Ve phenocryst texture microphenocryst texture
S p Loc glass Ve remarks
# wt% olivine plag spinel olivine plag cpx
Porphyritic; 6-7 mm clots of
Zoned, Tabular- Possibly ol. & plag.; intersertial g.m.
Skeletal, embayed, swallow quenched with opaques; alteration
2384-4B A-B ND Y Y embayed skeletal None Hopper tail in g.m. inside vesicals
Possibly Porphyritic; intersertial with
Rounded, Skeletal, zoned, Swallow quenched opaques; primarily plag.
2384-4C A-B ND Y Y skeletal sieve texture None Hopper tail in g.m. phenocrysts vs. ol.
Porphyritic; variolitic;
euhedral spinel
Many, 4-5 Tabular- microphenocrysts; 2
mm; skeletal, swallow populations of plag.
2384-6 A-B 9.57Y Few resorbed None Skeletal Hopper tail None microphenocrysts
Porphyritic; sparsely phyric;
Swallow variolitic with ol. and plag.
2384-7 A-B 9.9Y None Skeletal None Resorbed Yes tail None microphenocrysts
5-6 mm, Almost all glass with
embayed, variolites and ol.
rounded, Skeletal, variolites Variolitic microphenocrsts and
2384-9 A-B 9.73Y None skeletal None around around None None phenocrysts
Porphyritic; primarily glass
2-3 mm, with microphenocrysts;
rounded, rounded spinel
embayed, Inside and next to microphenocrysts; variolitic-
D20-6 A-B 9.87Y Few skeletal None olivine Dendritic Dendritic None dendritic with opaques
2-3 mm, Porphyritic; glass-variolitic-
D20-5 A-B 10.6Y None skeletal None Skeletal, zoned Hoppper Dendritic None dendritic
Poikilitic; intersertial with
6.91 Very large opaques; ol. and plag.
2379-2 A-B (WR) Y few Rounded Skeletal None Tabular Tabular None intergrown
Poikilitic; intersertial g.m.
6.91 4-6 mm, with large opaques; ol. and
2379-2 A-B (WR) Y Few embayed Sieve texture None Tabular Tabular None plag. intergrown
















Table 4-1. Continued.
Sample MgOlass Ve phenocryst texture microphenocryst texture
S p Loc glass Ve remarks
# wt% olivine plag spinel olivine plag cpx
Inside and outside Possibly
Rounded, olivine, skeletal, Swallow quenched Porphyritic; intergranular
2384-1 A-B 9.6N Y skeletal None embayed Tabular tail in g.m. with opaques
Dendritic -
Few, about 1 swallow Primarily microlitic; variolitic-
2384-10 A-B 9.59Y Y mm, rounded None Skeletal, zoned Dendritic tail None dendritic
Possibly Porphyritic; intergranular
Rounded, Zoning, skeletal, Few, quenched with opaques; large plag. &
2388-6 A-B ND None Few altered edges embayed None rounded Tabular in g.m. ol. clots 3-4 mm
Huge, resorbed Porphyritic; intergranular;
Embayed- edges, skeletal, Swallow Possibly microphenocrysts align
2388- subhedral, with very slight Rare, tail quenched around plag. Phenocrysts;
6WR A-B ND None Few huge zoning Inside olivines rounded tabular in g.m. plag & ol clots about 5 mm
Few, about 4
mm, embayed, Rare, Possibly
skeletal, broken up, quenched Primarily microlitic;
2388-7 A-B ND None Few None resorbed rims None embayed Subhedralin g.m. intergranular with opaques
Possibly
Tabular quenched Poikilitic; intersertial with
2391-10 A-B ND None Y None Embayed None Hopper anhedral in g.m. opaques
Possibly
Subhedral- quenched Poikilitic; intersertial with
2391-10 A-B ND None Y None Embayed None anhedral Tabular in g.m. opaques
Embayed, Inter-
skeletal, granular Tabular Possibly Porphyritic; intersertial with
Very few, zoning,sieve with swallow quenched opaques; clots of plag.
2391-6 A-B ND Y_ Y skeletal texture None plagioclase tail in g.m. phenocrysts
Acicular- Possibly
Skeletal, Skeletal, Hopper- swallow quenched Porphyritic; intersertial with
2391-7 A-B ND Y_ Y rounded embayed, zoning None subhedral tail in g.m. opaques; plag. clots 3-4 mm















Table 4-1. Continued.
Sample MgO glass Ve phenocryst texture microphenocryst texture
Loc glass Ve remarks
# wt% olivine plag spinel olivine plag cpx
Phorphyritic; mainly glass;
variolitic with hopper ol.
Embayed, Rounded, microphenocrysts and very
A25- skeletal, embayed, slightly Hopper- Very few, few dendritic plag.
D20-8 A-B ND Y__ None rounded None zoned edges dendritic dendritic None microphenocrysts

Rounded, skeletal, zoned Few, Phorphyritic; ol. & plag.
D-17B A-B ND Y ___skeletal Skeletal edges dendritic Dendritic None clots; dendritic
Possibly
Intergrown Skeletal, quenched Poikilitic; intersertial with
D17-4 A-B ND None few with plag embayed None Subhedral Subhedralin g.m. opaques; green alteration
Sparsely porphyritic; mainly
glass; variolitic dendritic
Skeletal, Lots, zoned rims, Very few; with spinel
D20-5 A-B 10.6Y few rounded None -skeletal Hopper dendritic None microphenocrysts
Swallow Possibly Porphyritic; variolitic -
Skeletal, Zoned rims, tail quenched dendritic with opaques;
D20-6 A-B 9.87Y Y rounded None skeletal Hopper dendritic in g.m. mainly hopper ol. in glass
Few, about 1 Swallow
Very mm, skeletal, tail Primarily microlitic; variolitic-
2384-11 A-B 8.79Y few embayed None None Subhedral dendritic None dendritic
Few, about 1 Swallow Primarily microlitic; variolitic-
mm, skeletal, tail dendritic; aligned plag.
2384-11 A-B 8.79Y Few embayed None None Subhedral dendritic None microphenocrysts
Intergown Possibly
Few, around with Swallow quenched Primarily microlitic;
2384-12 A-B 9.11 Y Y Few, skeletal None olivines plagioclase tail -in g.m. intergranular with opaques
Porphyritic; variolitic -
Swallow dendritic; ol. clots with
Slightly tail dendritic growth of plag
2384-13 A-B 8.53Y Y __ Skeletal None None hopper dendritic None around














Table 4-1. Continued.
Sample MgO phenocryst texture microphenocryst texture
Loc gglass Ve remarks
# wt% olivine plag spinel olivine plag cpx
5-6 mm, Possibly Porphyritic; intersertial with
skeletal, Skeletal, slight Swallow quenched opaques clots of ol. and
2384-14 A-B 7.23,None Y rounded zoning None Subhedral tail in g.m. plag. phenocrysts
Possibly
quenched Porphyritic; intersertial with
2384-14 A-B 7.23None y Skeletal Skeletal None Subhedral Tabular in g.m. opaques
Swallow
Skeletal, 5-6 Rounded, zoned Hopper- tail Porphyritic; intersertial with
2384-2 A-B 9.54,Y mm None rims dendritic tabular None fine opaques
Porphyritic; intersertial with
Rounded, Embayed, Possibly opaques; circular
embayed, in skeletal, zoning Slightly Swallow quenched alteration; alignment
2384-4 A-B ND Y Y clot in some Inside olivines hopper tail in g.m. around clots
Porphyritic; intersertial with
Skeletal, Subhedral- Swallow opaques; huge clots of
2384-4A A-B 8.35Y Y__ embayed Skeletal, zoning Inside olivines hopper tail None plag. & ol.
Swallow Porphyritic; intersertial
Embayed, Subhedral- tail g.m. with opaques; clots of
2384-4C A-B ND Y__Y Skeletal zoning Inside olivines hopper tabular None ol & plag 2-3 mm
Few, Possibly
Very few, skeletal, quenched
2388-1 A-B ND N Many None skeletal None rounded Tabular in g.m. Poikilitic; opaques in g.m.
Possibly
Few, Subhedral quenched
2391-9 A-B ND N irregular None Very resorbed None -anhedral Tabular in g.m. Poikilitic
Y, Resorbed, Possibly Porphyritic; intergranular
alteration skeletal, in Skeletal, Alteration around, Few, quenched with opaques; primarily
2391-8 A-B ND N around clots resorbed, zoned rounded subhedral Acicular in g.m. plag. in clots 2-3 mm
Skeletal, Few, Possibly
Skeletal, embayed, hopper- quenched Phorphyritic; green-brown
2391-2 A-B 7.84N Very few embayed zoning None subhedral Acicular in g.m. alteration; plag. clots
















Table 4-1. Continued.
Sample MgO phenocryst texture microphenocryst texture
Loc glass Ve remarks
# wt% olivine plag spinel olivine plag cpx
Skeletal, Swallow Porphyritic; variolitic -
Rounded, embayed, sieve Hopper- tail intersertial with opaques; 4-
2376-8 B 8.02Y _Y skeletal texture None dendritic tabular None 5 mm clots of plag. & ol.
Porphyritic; 3 mm clots of ol.
& plag.; variolitic dendritic
Skeletal, with opaques; 3 populations
2376-9 B ND Y Y embayed Zoned, skeletal Zoned rim Dendritic Dendritic -None of plag
Sieve texture, Possibly Porphyritic; intersertial with
6.68 rounded, zoned, Swallow quenched opaques; mainly plag. clots
2382-10 B (WR) Y__ None Skeletal skeletal None Tabular tail in g.m. with minor ol.
Skeletal, Porphyritic; variolitic -
Embayed, oscillatory intersertial with opaques;
2376-11 B ND N rounded zoning, huge None Subhedral Tabular None plag. & ol. clots about 4 mm
Sieve texture, Swallow Phorphyritic; intersertial with
2387-1 B-C ND Y__ Few Rounded zoning, skeletal None Tabular tail None opaques
Out of Porphyritic; 2-3 mm clots of
Y, equilibrium, Oscillatory Swallow large plag. with small ol. &
glass not as big as zoning, sieve Small, tail plag.; variolitic intersertial;
2387-2 B-C 7.56INone filled plag. texture, skeletal None subhedral tabular None brown and green alteration
Porphyritic; 2-3 mm clots of
Embayed, Subhedral Swallow large plag. with small ol. &
zoning, skeletal, euhedral, tail plag.; variolitic intersertial;
2387-2 B-C 7.56 Y_ Y Few sieve texture None skeletal tabular -None brown and green alteration
Porphyritic; 5-6 mm clots of
Few, Zoning, Possibly plag. & ol. phenocrysts and
embayed, embayed, sieve quenched microphenocrysts;
2387-5 B-C 7.79INone Y skeletal texture None Subhedral Tabular in g.m. intersertial with opaques
Zoning, Possibly
Very rare, embayed, sieve quenched Poikilitic; 5 mm plag. clots;
2387-7 B-C ND None Y rounded texture None Anhedral Anhedral in g.m. intergranular with opaques













Table 4-1. Continued.
Sample MgO phenocryst texture microphenocryst texture
Loc gglass Ve remarks
# wt% olivine plag spinel olivine plag cpx
Rounded,
Embayed, skeletal, sieve Hopper- Swallow Porphyritic; variolitic-
2385-1 C ND Y Few very few texture Zoned, skeletal tabular tail None intersertial
Rounded,
Embayed, skeletal, sieve Hopper- Swallow Porphyritic; variolitic-
2385-1 C ND Y__ Few very few texture Zoned, skeletal tabular tail None intersertial
Possibly
Acicular quenched Microlitic; intersertial with
D34-2 C-D 9.12 Y_ Y Few, skeletal Few, skeletal -None Skeletal tabular in g.m. opaques
Possibly Primarily microlitic; few plag.
Y, One about 1 Anhedral quenched clots; intersertial with
2386-3 D ND N many None mm, embayed -None subhedral Subhedralin g.m. opaques
Swallow Possibly
Y, Few, small Anhedral tail quenched Primarily microlitic; few plag.
2386-6 D ND N many None embayed None subhedral tabular in g.m. clots
Possibly
Anhedral quenched Porphyritic; plag. clots;
2386-8 D ND None Y None Clots None subhedral Tabular in g.m. intersertial with opaques
Possibly Microlitic; intergranular with
One about 1 quenched ol., plag., and opaque
2386-1 D ND Y___ None mm, embayed None Subhedral Subhedralin g.m. microphenocrysts
Primarily microlitic;
Skeletal, Acicular intersertial with opaques;
2390-6 VWRTI ND Y Y Skeletal embayed None Hopper dendritic None primarily plag.


Notes: ND = no data; Ve = vesicals.
























C. D.














Figure 4-1. Photomicrographs taken under plain light (a & b) and cross polarized light (c & d). Picture width is equal to
approximately 1.8 mm. A) Rounded and embayed olivine phenocrysts surrounded by glass and variolites (sample 2384-7).
B) Olivine microphenocrysts in transition from variolitic to dentritic texture (sample 2384-7). C) Plagioclase
microphenocrysts growing around olivine glomerophenocryst in sample 2384-13 a picritic basalt. D) Plagioclase and
olivine phenocrysts in intersertial groundmass with opaques and quenched clinopyroxene (sample 2391-10).


























C. D.














Figure 4-2. Photomicrographs taken under plain light (a, b, & d) and cross polarized light (c). Picture width is equal to approximately
1.8 mm. A) Edge of large plagioclase with very resorbed edge surrounded by olivine, plagioclase and opaque
microphenocrysts (sample 2387-7). B) Rounded olivine phenocrysts in dendritic groundmass with hopper olivine
microphenocrysts (sample 5)0-5). C) Plagioclase phenocrysts with oscillatory zoning in variolitic groundmass with
olivine and plagioclase microphenocrysts (sample 2389-1). D) Rounded olivine phenocrysts with spinel inside attached in
variolitic groundmass (sample 2384-3).
























C. D













Figure 4-3. Photomicrographs taken under plain light (c) and cross polarized light (a, b, & d). Picture width is equal to approximately
1.8 mm. A) Plagioclase and olivine clot in intersertial groundmass of plagioclase and olivine microphenocrysts (sample
2384-4). B) Rounded olivine phenocrysts in intersertial groundmass of olivine and slightly swallow tail plagioclase
microphenocrysts (sample 2384-1). C) Circular variolites with olivine microphenocrysts (sample 2384-9). D) Small
olivine plagioclase clot within intersertial groundmass of plagioclase, olivine, and quenched clinopyroxene (sample 2384-
4).









of vesicularity than EPR basalts usually found between 2500m to 2800m depth. Several

dredges were targeted to sample the cone-shaped features in the axis of the A-B fault.

The position of these features, roughly halfway along the fault, suggested they would be

located in older volcanic terrane; but their morphology suggested recent constructional

volcanism. Dredge D-20 started in an area of small cones near the midpoint of the A-B

fault and proceeded up the south wall of the fault valley. Large quantities of young-

looking glassy basalts, many of which were picritic were recovered in the dredge. Three

additional dredges (D-22, D-23, and D-24) were carried out in the deep areas of the A-B

fault, on small, closed-contour peaks and saddles in the axis of the trough, and up the

middle to lower walls of the fault valley (Figure 4-4). Young, glassy pillow basalts were

found throughout the A-B fault. To more completely document the loci of eruption and

tectonic setting of the young-looking, olivine-rich basalts Alvin Dive 2384 traversed the

floor of the A-B fault and went up the south wall across on the cone-like features (Figures

4-5). The basalt-floored depression where dive 2384 began was found to be nearly

devoid of sediment which is unusual for transform faults. Fresh olivine-phyric basalts

were recovered from a volcanic slope characterized by intermingled pillows, broken

pillows, and fragments of basalt all fresh and glassy. White to yellow staining, inferred

to be hydrothermal in origin, was abundant on the broken basalt surfaces. Another field

of fresh basalt flows was found along the base of the south wall of the fault valley. Here

nearly intact pillows and tubes were found. The north side of the fault axis consisted of

rugged constructional volcanic terrane with occasional steep-sided (up to 700), flat-

topped, volcanic "haystacks". Glassy picritic basalts with little or no Mn coatings were

recovered from a free-standing cone. Around the cone, the seafloor was built of glassy








40



2500m



3000m



3500m

"B" Axis

4000m



25'N









Southern
"A" Axis 2500m

























824'N l03 40'Wtw



8" 2T'N

103' 42'W



Figure 4-4. Dredge and Alvin dive locations within the A-B fault.











Distance (meters)


500


1000


1500


3500



3600


1:30


12:00


12:30
Time


13:00


13:30


3000


Distance (meters)
2500


2000


Lower North Wall of A-B Fault ....... ------suB. DEPTH
. ... .... ...... ... .. ...... .. ..... 1 3 ...... .. ........................ ......... .......................... ......... .......................... ...........................
14 -- TOTAL DEPTH -


.......O l .e .a a ....... ................. ... .............. .. ................................................................. .................. .............


Older Lava
... .. .. .. .. ... .. .. .. .. ... .. .. .. .. .......................................^ .l .......... ........ ........ ........ ........ ........ ........................................................................................... .

nee~~ 10

"8 L


Y young Lavas ........ .
..... .. ........ .. ......... .. ......... .. .... .. .. .. . .... .. ......... .. .. .. ................... .. ......... .. ........
O l d e r ... .... .. .. .. ... .... .......3 J.. .. 1.. ... .. .. ..... ... .. .. ... .. .. .. .. ...... ^ . if ... _... .. ... ............ ... .......... ... ........... .
.............................................................. .................................................................................................... .. ^ .. .. .. .. ............................... ...

........................................................................... %' .. .. .. ... .. ...N.. .. ........................... 1 0 .. ..... .... ......... ......... .... ......... .. ........... .^ ^. ^.... .. ^... .. .. .


..... ..... O/C 327 ....... ........


3800 -f-
15:30


15:00


14:30
Time


14:00


13:30


Figure 4-5. Alvin dive 2384 traverse.


-----SUB. DEPTH

T TOTAL DEPTH Lower South Wall of A-B Fault ...............




.Fault Axis...........................%..........

...3 .........
S : Older Lavas: :'S






young Lavas
/C 21 C/C 237
........... ::::::::.::::::::::4::::::::.:.::::::::.:.:::::.:.::::::::.:.::::::::.:.:::::::.::::::::.:.:......:..:........:..:........:........:..:........:..:....................4...........,.......:


D
e
P
t
h
(m)


3700



3800


3900



4000
1


3300



3400



3500



3600


3700









basalt flows and veneered with basalt blocks and glass shards. To the north, a

bathymetric notch separates the younger volcanic terrane of the south from an older

terrane to the north (Figure 4-5). Here sediment was more abundant and glass is absent.

Only plagioclase-phyric or olivine + plagioclase-phyric basalts were recovered from the

older lava flows.

Within the 28 km long A-B fault, 9 cone like features believed to be constructional

volcanic features have been identified (Wendlandt and Ridley, 1994). Bulbous to

elongate pillows are the dominant basalt morphology within the A-B fault, similar to that

observed along intra-transform spreading centers, but in marked contrast to the lava

morphology at the EPR axis, where sheet flows and lobate forms dominate (Ballard et al.,

1981; Kastens et al., 1986; Perfit et al., 1991). The erupted lavas overflow a severely

tectonized terrane on the valley walls, but the young flows from which the olivine-rich

samples were recovered had little structural disruption of the flow surfaces. The olivine

rich basalts recovered from the A-B fault valley are inferred to have erupted recently as

evidenced by the extreme freshness of the glassy lava surfaces, thin to non-existent

sediment cover, and relatively minor structural disruption of the flow surface. The

youngest-looking basalts are as glassy as young lavas that floor the axial summit caldera

on the EPR between 930'-54' N (Haymon et al., 1991, 1993; Fornari et al., 1991; Perfit

et al., 1991).

Siqueiros Sample Petrography

The extremely olivine-rich picrites and picritic samples which lack plagioclase

phenocrysts were only recovered within the A-B fault. The older looking, Mn-encrusted

basalt samples from the talus and sediment-covered terrains surrounding the fresh flows

are plagioclase + olivine spinel-phyric, or plagioclase spinel-phyric (Perfit et al.,









1996). The samples from the other faults, the three spreading centers, tough D, and the

RTIs are also plagioclase + olivine spinel-phyric, or plagioclase spinel-phyric. Most

samples are porphyritic, containing phenocrysts and microphenocrysts.

The olivine-rich samples tend to be hypohyaline having glassy margins with sparse

microphenocrysts and variolitic or dendritic interiors. The plagioclase phyric samples

range from hypohyaline to hypocrystalline or holocrystalline with nearly completely

crystalline interiors and margins and only minor glass in the groundmass. Olivine and

plagioclase microphenocrysts are present in almost all of the samples and a few samples

had microphenocrysts of spinel. The more crystalline samples have opaques in the

groundmass. Clinopyroxene phenocrysts have not been identified, but in the more

crystalline samples clinopyroxene appears as a quenched phase in the groundmass.

Olivine microphenocrysts are commonly found in centers of variolites or with dendritic

growth of plagioclase surrounding them. Olivines range from hopper crystals in the

variolitic samples to subhedral in the more crystalline samples. Plagioclase

microphenocrysts usually have swallow tail to dendritic forms. Euhedral and subhedral

tabular plagioclase crystals are abundant in the more crystalline samples.

In all the samples, including the olivine-rich samples from the A-B fault, the large

phenocrysts have textures indicating that they were not in equilibrium with their host

melt. Subhedral to euhedral olivine phenocrysts vary from 1 mm to 7 mm in size, but

tend to have rounded or embayed edges and are often skeletal. Many of the samples are

glomeroporphyritic with clots of olivine and plagioclase growing together. Larger

plagioclase phenocrysts are commonly glomeroporphyritic and have skeletal, rounded or

embayed edges. Many of the larger plagioclase phenocrysts also exhibit oscillatory









zoning and some have sieve or moth-eaten textures. Spinel phenocrysts exist inside and

outside olivine phenocrysts. The spinels were usually rounded, sometimes skeletal, and

typically red to brown with darker rims. Clinopyroxene is absent from all samples, which

is unusual for MORBs. However, because most of the thin sections were made of the

samples from the A-B fault, the samples are fairly primitive and clinopyroxene is not

expected on or near the liquidus of more primitive samples at low to moderate pressures.

Crystal Liquid Equilibria

In addition to thin sections, elemental analyses was completed by microprobe on

olivine, plagioclase, and spinel crystals (Appendix B). The olivine microprobe analysis

was done on small, medium, and large olivine phenocrysts. A comparison of the Mg#

(Mg2+/ (Mg2+ + Fe2+)) of the glass surrounding the olivine and the Fo content of the

olivine shows a strong correlation (Figure 4-6). The total compositional range in the

Siqueiros basalts is from Fo90o.9 to Fo8o.o, which is a slightly greater range than that found

in olivine microphenocrysts and microlites within MORBs from the East Pacific Rise at

9030'N. Olivine phenocrysts in the 9030'N lavas have been found to range from Foss to

Fo82 (Pan and Batiza, 2003). With successive fractional crystallization the Mg# of the

melt and the Fo content of the crystallizing olivine both decrease. In the Siqueiros

sample suite, there does not seem to be a correlation between the size and Fo content of

the olivine. Although the majority of the large olivines have more forsteritic

compositions, some of the large olivines have lower Fo contents and some of the small

olivines have relatively high Fo contents. For the larger olivine phenocrysts, successive

microprobe analyses were done from the center outward to the rim. Individual crystals

have little chemical zonation. Some olivines show slight reverse zoning with the Fo

content increasing from the core to rim (2384-3-olh), while other samples are normally












Core of small olivine
Interior of small olivine

Core of medium olivine
Interior of medium olivine

Core of large olivine
Interior of large olivine

2377-7
+ D34-2
A 2384-9
o 2384-9 High Pressure
75
70

65 65
60 -
'- 55
50 -
45 -
40
35
70 75 80 85 90
Fo content of olivine



Figure 4-6. Comparison of olivine forsterite content with the Mg# (Mg2+/(Mg2+ + Fe2+))
of the host glass. Modeled equilibrium trends expected during fractional
crystallization were calculated using the low pressure model of Danyushevsky
(2001) for three different parental compositions. High pressure model of
Danyushevsky (2001) also shown for comparison.

zoned and the Fo content decreases slightly from the core to rim (2388-3a-oll). Liquid-

mineral equilbria for olivine (Mg# versus the Fo content) were calculated using the

method of Danyushevsky, (2001) (Figure 4-6). Slightly different results were obtained

using the ol-liquid equilibria model of Herzberg & O'Hara (2002). Fo-liquid equilbria

were determined for three different parental liquids as they fractionally crystallized using

the Petrolog program. There is almost no difference for the three parental composition

used in the major element models and there is no difference between the high and low


models and there is no difference between the high and low









pressure models. The Fo content of olivine steadily decreases as the Mg# of the host

magma decreases and as the olivine-melt partition coefficient (Doi-melt- Feol MgL/Mgol *

FeL) increases slightly with decreasing temperature. The Herzburg & O'Hara (2002)

model calculations predict slightly lower olivine partition coefficients, which fit the

observed data better (Figure 4-7). The olivine-melt partition coefficient in MORB lavas

has been found to range from approximately 0.31 to 0.28 (Pan and Batiza, 2003).

Olivine-liquid pairs in Siqueiros samples fall on the low side of this range of partition

coefficients, requiring Kds of less than 0.28 if the olivine phenocrysts are actually in

equilibrium with the host magma (Figure 4-8), alternatively, the olivines may be

xenocrystic. The more Fo compositions of the Siqueiros olivines suggests that many of

the olivine phenocrysts came from more Mg-rich melts and are out of equilibrium with

the host magma. This is also suggested by the embayed edges and skeletal textures found

in many of the olivine phenocrysts.

Chemical analysis of plagioclase by microprobe shows that the plagioclase

phenocrysts have much greater compositional variations than olivine phenocrysts

(Appendix B). Analyses were completed on small and large plagioclase crystals and

include core, rim, and interior zones. The total compositional range of plagioclase

phenocrysts is from An5s.o to An88.3, compared to a composition range of An52.1 to An83.4

for phenocrysts from 903'N on the EPR (Pan & Batiza, 2003). The average Siqueiros

plagioclase is slightly more calcic (An75.9) than the average plagioclase from 9030'N on

the EPR (An685). Many of the large plagioclase crystals show visible zoning. Core to

rim analysis show that the zones exhibit oscillatory zoning (Figure 4-9). Overall, the













Core of small olivine
Interior of small olivine
Rim of small olivine
Core of medium olivine
O Interior of medium olivine
Rim of medium olivine
Core of large olivine
O Interior of large olivine
Rim of large olivine
A 2377-7
D34-2
2384-9
+ 2384-9 High Pressure
75

70

U 65
U)
(0 60
0
N 55

50

45

40
70 75 80 85 90
Fo content of olivine

Figure 4-7. Comparison of Olivine forsterite content with the Mg# (Mg2+/(Mg2+ + Fe2+))
of the host glass. Modeled equilibrium trends expected during fractional
crystallization were calculated using the low pressure model of Herzburg &
O'Hara (2002) for three different parental compositions. High pressure model
of Danyushevsky (2001) also shown for comparison.






48




80

K (meltlol) = .32
0.30
0.28


70



65



60



55



50



45



40
75 80 85 90 95
Fo content of olivine


Figure 4-8. Calculated Fo contents of olivine for partition coefficients ranging from 0.28
to 0.32. Siqueiros samples fall on the low side of the acceptable olivine-melt
partition coefficients indicating that they are from more mafic magmas than
that of the host glass.













2377-11, plagioclase 11


74

72
0
CORE


20 40 60 80 100
RIM


Figure 4-9. An contents for core, interior, and rim locations in Siqueiros plagioclase
phenocrysts.












Core of Small Plag
Interior of Small Plag
0 Rim of Small Plag
Core of Large Plag
Interior of Large Plag
Rim of Large Plag
A 2377-7

+ 2384-9
A 2384-9 High Pressure
90
sE 85
80
C 80 -
0
C 75
0 70 i

0 65
0) 60


50 A
4 5 6 7 8 9 10 11
MgO



Figure 4-10. Comparison of plagioclase An content from Siqueiros samples and An
content evolution for three of the major element parental compositions.
Model equilibrium trends expected during fractional crystallization were
calculated using low pressure model of Danyushevsky (2001).

smaller plagioclase laths have lower An contents (Avg. An content = 69.9) than the larger

laths (Avg. An content = 78.3). Anorthite contents of the small plagioclase phenocrysts

exhibit a decrease with magma evolution (decreasing MgO) as predicted in the fractional

crystallization models (Figure 4-10). Larger plagioclase phenocrysts and a few of the

smaller phenocrysts have higher An contents (for a given MgO or CaO) than predicted

and have large variations in An content from core to rim (core-rim), suggesting that the

phenocrysts are out of equilibrium with the host glass.

The relationship between An content and the Ca# (Ca/(Ca + Na)) of the magma is

complicated. Calculated plagioclase-liquid equilbria indicate that the Ca# initially


ly









decreases slowly with fractionation, but once clinopyroxene begins fractionating from the

liquid the Ca# decreases rapidly. Models that describe the relationship between the Ca#

of the magma and the coexisting plagioclase An content were produced using the

alogrithims of Danyushevsky, (2001) and Langmuir et al. (1992) (Fig. 4-11 and 4-12).

The best fit to the observed data were produced using the plagioclase-liquid equilbria

model of Danyushevsky, (2001). The compositions of most of the small plagioclase

crystals follow the general trend predicted for fractional crystallization, suggesting

plagioclase compositions are controlled by the evolving magma chemistry. Large

phenocrysts have higher An contents than predicted by the model (Figure 4-11). The

higher An contents suggest that the phenocrysts Ca# content. Such high Ca# plagioclase

phenocrysts have been found by others, but high Ca# melts are not commonly found in

MORB (Ridley et al., in prep; Pan and Batiza, 2003).

Chemical analyses of cores of small spinel phenocrysts and cores, interiors, and

rims of the larger spinel phenocrysts are presented in Appendix B. The Cr# (100*Cr/(Cr

+ Al)) of the spinels range from 25-58 and generally decrease from core to rim (Figure 4-

13). The Fe203 content (2.08-7.07 wt. %) and TiO2 content (0.12-0.98 wt. %) of the

spinels are low; similar to other spinels from MORB (Dick and Bullen, 1984; Allan et al.,

1988). The Fe3+/(Cr +Al + Fe3+) vs. Fe2+/(Mg + Fe2+) compositions of the Siqueiros

spinels fall within the range and have trends observed in spinels from other MORB lavas

(Figures 4-14). The Siqueiros spinels generally follow the Cr-Al trend observed in other

spinel suites, but the larger phenocrysts in particular, have low Fe2+/(Mg + Fe2+) for a













Core of Small Plag
Interior of Small Plag
o Rim of Small Plag
Core of Large Plag
Interior of Large Plag
Rim of Large Plag
A 2377-7

2384-9
A 2384-9 High Pressure
80


75 -


S70 2,
70


c 65


60 -


55
50 55 60 65 70 75 80 85 90
Plagioclase An content




Figure 4-11. Comparison of the host glass Ca# (100*Ca/(Ca + Na) with the plagioclase
An content. Modeled equilibrium trends expected during fractional
crystallization were calculated using low pressure model of Danyushevsky
(2001). Modeled trends fit the many of the smaller plagioclase crystals, but
are unable to explain many of the samples that have high An contents
compared with the Ca# of their host glass.






























.

*:** oo*
* *** **


50 55 60 65 70 75
Plagioclase An content


80 85 90


Figure 4-12. Comparison of Siqueiros plagioclase An content vs. glass Ca# (100*Ca/(Ca
+Na)). Modeled equilibrium trends expected during fractional crystallization
were calculated using the low pressure model of Langmuir et al. (1992). High
pressure model is also shown for comparison. The Langmuir et al., 1992
m model does not provide a good fit to any of the Siqueiros plagioclase
compositions.


* Core of Small Plag
* Interior of Small Plag
Rim of Small Plag
* Core of Large Plag
O Interior of Large Plag
Rim of Large Plag
2377-7

+ 2384-9
A 2384-9 High Pressure


~*1
-S
*1~
0


*' 0 0


s**


4ED














55
D22-3, spinel 2

50

+

45



40



35
5 D22-3, spinel 4
50

48

+ 46

5 44

I 42

40

38 -

36
D20-8, spinel 1

27


+ 26
y. .r--^ ^----- ,
) 25
o

24


23
D21-1, spinel 1
55

50 -

45 -

o 40 -

35

30
0 20 40 60 80 100
CORE RIM



Figure 4-13. Spinel Cr# for core, interior, and rim locations.










1

0.9

0.8

0.7

0ft 0.6
o0.6 FeTi trend

0.5
U-
0.4
U-
0.3

0.2

0.1

0
0 0.2 0.4 0.6 0.8 1

Fe2+/Mg+Fe2+

Figures 4-14. Fe3+/(Cr + Al + Fe3+) vs. Fe2+/(Mg + Fe2+) plots for tholeiitic basalts.
Fields are from Barnes & Roeder, 2001 and enclose 50% (dark shading) and
90% (light shading) of the MORB data points.











Rim of large spinel
Interior of large spinel
Core of large spinel
Rim of medium-small spinel
Interior of medium-small spinel
Core of medium-small spinel






0.8 CrAl trnd




0.6
+








0.2




0
0 0.2 0.4 0.6 0.8 1

Fe2+/(Mg + Fe2+)


Figure 4-15. Cr/(Cr + Al) vs. Fe2+/(Mg + Fe2+) plot for tholeiitic basalts. Fields enclose
50% (dark shading) and 90% (light shading) of the MORB data points.
MORB fields are from Barnes & Roeder, 2001.









given Cr/(Cr + Al) when compared to other MORB spinels (Figure 4-15). Comparison of

the host-glass compositions with spinel compositions shows a strong correlation between

the Al content of spinel rims and glass, but not for core and interior compositions (Figure

4-16). Similar correlations between Al content of the host rock and spinel have been

found in other MORB lavas (Sigurdsson and Schilling, 1976, Allan et al. 1988, Dick and

Bullen, 1984). A strong correlation also exists between the Mg# of the host glass and the

Mg# of the spinels, with the strongest correlation observed for rim compositions (Figure

4-17). The Cr# of the Siqueiros spinels is independent of the host MgO content (Figure

4-18) as found in the Lamont Seamounts (Allan et al., 1988) but contrary to the results of

Irvine (1976). Some small spinels are present in the groundmass glass. These show Mg#

and Cr# correlations similar to those observed in the larger spinels attached or enclosed in

olivines (Figure 4-19). There was no correlation between glass Mg# and spinel Cr# for

either type of spinel phenocrysts (Figure 4-20).













+ 3.9306x R = 0.75193


7

6
8.5 9 9.5 10 10.5 11
Glass Mole % Al



Figure 4-16. Molecular percentage aluminum in glass versus molecular percentage
aluminum in spinel. There is a fairly linear relationship for rim compositions
of the spinels, but the there is less of a relationship for core and interior
compositions.




d inter Core of spinel
+ Interior of spinel
B Rim of spinel y = -0.45953 + 1.736x R2= 0.9263
0.875


0.75 2 1


0.7


0.65


0.6 0.62 0.64 0.66
Glass Mg#


0.68


0.7 0.72


Figure 4-17. Comparison of the composition of the cores, interiors, and rims of spinels
found in the groundmasses and within olivines with the composition of the
host glass. A strong correlation can be seen between spinel and glass Mg#.


I


can be seen between spinel and glass Mg#.


I











* Core of spinel
* Interior of spinel
[ Rim of spinel


0.66 0.68
Glass Mg#


0.7 0.72


Figure 4-18. Comparison of the composition of the cores, interiors, and rims of spinels
found in the groundmasses and within olivines with the composition of the
host glass. There is a poor correlation between host glass and spinel Cr/(Cr +
Al).


s Spinel inside or attached to olivine
Spinel in glass

y = -0.35923 + 1.5788x R2= 0.85569
y =-0.42799 + 1.7013x R2= 0.85135





1
*


0.6 0.62 0.64 0.66 0.68


0.7 0.72


Glass Mg#


Figure 4-19. Comparison of the composition of the spinels found inside olivines and
spinels found in the glass with the composition of the host glass. A strong
correlation between spinel and glass Mg# exists for both types of spinels.














EB Spinel inside or attached to olivine
* Spinel in glass


*


0.66 0.68
Glass Mg#


0.7 0.72


Figure 4-20. Comparison of the composition of the spinels found inside olivines and
spinels found in the glass with the composition of the host glass. There is
poor correlation between host glass Mg# and spinel Cr/(Cr + Al) for both
types of spinels.


0.62














CHAPTER 5
MAJOR AND TRACE ELEMENT CHEMISTRY

Major Element Trends

The major element compositions of Siqueiros basalt samples are presented in

Appendix C and are shown in Figures 5-1 and 5-2. For comparative purposes, the

samples in Figures 5-1 and 5-2 are divided into groups according to the geologic setting

from which they were recovered. Basalt samples analyzed in this study include samples

from the three spreading centers (A, B, and C), trough D, the 3 connecting transform

faults (A-B, B-C, and C-D), the western ridge transform intersection (WRTI), and the

eastern ridge transform intersection (ERTI). The lavas from the Siqueiros transform

domain can be classified as tholeiitic basalts having low K20 and total alkali contents as

well as showing FeO enrichment trends with decreasing MgO; characteristic of tholeiitic

suites (Tilley, 1950). The MORB recovered include picrites, picritic basalts, basalts,

ferrobasalts, and a few Fe- and Ti-enriched (FeTi) basalts. Classification of a ferrobasalt

is defined as containing greater than 12 wt. % FeO, but less than 2 wt. % TiO2, while

FeTi basalts are defined as containing greater than 12 wt. % FeO and TiO2 contents

greater than 2 wt. %. The MORB can be further divided into N-MORB (normal,

incompatible element-depleted mid-ocean ridge basalts), D-MORBs (exceptionally

depleted, incompatible element-depleted mid-ocean ridge basalts), E-MORB

(incompatible element-enriched mid-ocean ridge basalts), and T-MORB (mid-ocean

ridge basalts transitional between N-MORB and E-MORB). There is currently some

debate over the nomenclature of high-MgO volcanic rocks (Kerr and Arndt, 2001;

























----------- --- ----- .. ..----------- -----------L----------- .--------- --.......




----------...-----------......... ........... .---..----....................-------------------..........--
A






........... ........... ........... ...-?......^... -------.......


3


2.5


2
N
O0

1.5


1






51


50
N


49


48


47


A Spreading Center A
D A-B Fault
A Spreading Center B
B-C Fault
-:* A Spreading Center C
0 C-D Fault

'6 Ridge Transform Intersection

.................. ------.------------.-------------------
- --------- -----:--~rT~n-----2f===== ---- i====-



I II I b 1
4k []A

....-----------. ---- .------------- --


-----------.----- ---- -----t--1:----------- -------------

---:9------------i ---- l




.^ ---------. ------------ -.. -- ......... .. . .. . .



*A
IP 40
- -- A -- -- - -- -- -


5 6 7 8 9 10 11 5 6 7 8 9 10 11
MgO MgO


Figure 5-1. Major element variation diagrams for glasses from the Siqueiros transform domain. Samples are distinguished according
to their geologic locations within the transform. Picritic basalts and picrites are not shown on this diagram.


~-..---.- -... --+- --------------------------- -------


.. ................. v..^... ... .... ....................



------------ .----------------.- .------. .....-E.-- --- -----------

-- ... .................... ....................-


3.2


3


2.8


2.6 2)
0

2.4


2.2


13


12
0

11


10


9



























--------------------------^ ---------------------

----------...............--------- ---- -------- ------------ --- I -----
........... .......... -...................... ...........*........... ..........




.----------- .--- ------ ... ........ ............. .



-- .---- -
-- - -------


-----------...........i.......... ........... ........... ...... -........... -------

----- ---------- ---..........---. ... .......... ...
-----------*-I ------ -- ----A ---- - -

-- -- -- -- -- -- -- -- -- ---I ---- ------------
".......... ........ ....................... ..........


-------------- ---- --------- A --------------------- L----------- ----------
----...........-.....------ ....------------ ---------- ..................... ........... ---------..........

-.... .... T........... .......... ............. ........... ,........-.. -. .. .. .
-- - - -- -- I --- --- --


A Spreading Center A
FE A-B Fault
A Spreading Center B
B-C Fault
A Spreading Center C
E C-D Fault

..............,.......... Ridge Transform Intersection





0~c -- - -
----- ----- ----------- ------------ ----------- ---------- ----------- ------------



- -------- ------ ---------



..................... .... .....

; 6 n A a Daa


- ------ ------



........... ........... .......... ........... ............ .......... .........-


----------- ------ ---------------------

---------- ----------- ----------.---------------------------------- ----------

...................... ........... ........... .......... ........... ..........
-.... -..------------ --- ----------- -------


10 11


18

17

0o 16

< 15

14

13

12


0.5



0.4



0.3



0.2



0.1


0.7

0.6

0.5

0.4

0.3

0.2

0.1

0


Figure 5-1. Continued.


5 6 7 8 9 10 11 5 6 7 8 9
MgO MgO




































SA /$4
- -- -
AZL


*


A-


'A
A El



A, ----------
^%'A


10 11


MgO


Figure 5-1. Continued.


A Spreading Center A
o A-B Fault
A Spreading Center B
* B-C Fault
A Spreading Center C
* C

* Ridge Transform Intersection


0.1


0


0.3

0.25


0.2

0.15

0.1

0.05

0






















- -- -
4.


-ti


mm E


A Spreading Center A
: A-B Fault
A Spreading Center B
* B-C Fault
A Spreading Center C
* C-D Fault

* Ridge Transform Intersection
* Picrites and Picritic Basalts


Si :uni II ___
i i I | ,i i I


10 15 20 5 10 15 20


MgO


Figure 5-2. Major element variation diagrams showing the Siqueiros picrites and picritic basalts relative to more evolved MORB as in
Figure 5-1. Picrites and picritic basalts were only recovered within the A-B fault.


v-


3.2

3

2.8

2.6 z

2.4 "O

2.2

2

1.8



13

12

11 w

10

9


1;.


-


MgO































9

8

7



18

17

0 16

< 15

14

13

12


10 15 20 5 10
MgO


Figure 5-2. Continued.


0.5


0.4


0.3
IO
0
0.2


0.1


0.7

0.6

0.5

0.4 7
0
0.3

0.2

0.1

0


MgO














A Spreading Center A
o A-B Fault
A Spreading Center B
B-C Fault
A Spreading Center C


Ridge Transform Intersection
Picrites and Picritic Basalts


0.4 |-------------------E)--------------
0.5


0.4 -


0.3 -


0.2 -
C-
46
0.1 -
**

A -----




0.25-




s 0 .15 -- --- A--- -1^- ------ - ---- ----- ---- ---- ------- -- -





'------------------ ------------------ --'---
O 0.2 --










5 10 15 20
MgO

MgO


Figure 5-2. Continued.









Le Bas, 2001). The IUGS classification (Le Bas, 2001) for a picrite is >18 wt% MgO

with between 1 and 2 wt0/o total alkalis. This definition is based entirely on the chemistry

of the rock. Others (Kerr and Arndt, 2001) advocate for a definition that places greater

emphasis on the texture of the rock, which reflects the conditions of crystallization. Such

definitions require an abundance of olivine phenocrysts in order for a rock to be classified

as a picrite. Of the rocks analyzed for this study, only two can be classified as picrites by

the IUGS classification. These two rocks are also rich in olivine phenocrysts and fit into

the textural definition of a picrite. The term picritic basalt is used to describe highly

magnesian rocks that are also olivine phyric, but have MgO contents too low (12-18 wt%

MgO) to be classified as picrites. The picrites and picritic basalts have only been found

within the A-B fault.

Where trace element analyzes are not available the ratio of K20 to TiO2 can be

used as a proxy to discriminate between depleted (Ce/Yb < 1) and enriched (Ce/Yb > 1)

samples (Perfit et al., 1994.) In the database for the 90-100N segment of the EPR there is

a natural break at K20/TiO2 = 0.11. Samples with K20/TiO2 values < 0.11 are

considered N-MORB, which have normal incompatible element-depleted signatures

(Smith et al., 2001). The majority of samples recovered from the 90-100N area are N-

MORB, but a small percentage (-15%) of samples found off-axis (300-500 m) have

values > 0.11 and are transitional to incompatible element enriched basalt (T-MORB or

E-MORB). For the 11-12' segment of the EPR, the depleted versus enriched sample

break was found to correlate with a K20/TiO2 value of 0.25 (Hekinian et al., 1989). The

K20/TiO2 ratios of the Siqueiros samples are shown in Figure 5-3. In the Siqueiros

samples there is a break between K20/TiO2 values < 0.10 and K20/TiO2 values greater














A Spreading Center A
o A-B Fault
A Spreading Center B
B-C Fault
A Spreading Center C


Ridge Transform Intersection
EPR Field

0.4


O 0.3


S0.2- -


0.1




5 6 7 8 9 10 11 12
MgO

Figure 5-3. Comparison of K20/TiO2 of Siqueiros samples with samples from the EPR. A K20/TiO2 > 0.11 indicates an enriched
sample. The Siqueiros samples are very depleted when compared to the EPR field.









than 0.15. All of the samples with K20/TiO2 values greater than 0.15 are from the RTI

and the group includes sample 2390-1, which is well documented to be incompatible

element enriched as well as to have higher 17Sr/86Sr than all other MORB from the region

(Lundstrom et al., 1999). For the Siqueiros sample suite, K20/TiO2 values < 0.11

correlate with depleted Ce/Yb ratios (Discussed in Trace Element Trends section).

Unlike the EPR, none of the Siqueiros samples have transitional K20/TiO2 values

between 0.15 and 0.25 (Figure 5-3). Compared to the EPR, Siqueiros samples are

significantly more depleted and include very few enriched samples. Only the samples

from spreading center B that have higher K20/TiO2 values and the enriched RTI samples

overlap with the EPR field.

The Siqueiros samples have a narrow range in MgO content with a relatively

primitive average of 8.31 wt. % MgO. The most primitive MORB are found within the

A-B fault (MgO contents of 10-10.5 wt. %), but were recovered near basalts that had

MgO contents of -7 wt. %. The most evolved MORB are found near the RTIs (MgO

contents of 5.38 wt. %) and were recovered with samples that have as much as ~8 wt. %

MgO. The most primitive samples have FeO, and TiO2 contents (in wt. %) of 7.13% and

0.93%, respectively and the most evolved samples have FeO and TiO2 contents of

12.79%, and 2.85%, respectively.

Most of the variation seen in major elements on the segment scale is due to low-

pressure crystallization in shallow magma chambers (Perfit et al., 1983; Langmuir et al.,

1992; Batiza and Niu, 1992). Low-pressure crystallization results in changes in melt

compositions that vary systematically with MgO, which has been shown to decreases

during cooling due to the removal of olivine from the melt (Langmuir et al., 1992). The









basalts from the Siqueiros transform exhibit major element variations that appear to

mainly reflect the effects of crystal fractionation (Figures 5-1 and 5-2). In the Siqueiros

sample suite, FeO and Na20 contents steadily increase with decreasing MgO content.

P205, K20, and TiO2 also increase with decreasing MgO, but to greater relative extents.

MnO and SiO2 show a little more scatter, but also generally increase with decreasing

MgO. Cr203 decreases with decreasing MgO and CaO and A1203 both initially increase

and then decrease with decreasing MgO. These variations are compatible with initial

olivine fractionation, followed by plagioclase fractionation, and finally clinopyroxene

fractionation (Batiza et al., 1977; Perfit et al., 1996).

Comparison of the 3 spreading centers shows that the major element variations of

lavas from the 3 spreading centers are generally very similar. Spreading center B does

contain samples that are slightly more evolved (MgO < 7%) and has some samples with

slightly higher TiO2, A1203, P205, and K20 for a given MgO when compared to the other

spreading centers and the faults. Spreading center A has a group of samples with lower

Na20 for a given MgO content when compared with all other Siqueiros samples (Figure

5-2). The most primitive samples were recovered from the A-B fault and all of the

picritic basalts were found within this fault. The other faults, B-C and C-D, also contain

some samples that are relatively primitive in comparison to the spreading centers. When

compared to the other localities within the Siqueiros transform, samples from the B-C

fault have low TiO2, K20, FeO, and P205 values. Fault B-C has the most evolved

samples of the 3 intra-transform shear zones. The most evolved samples from the entire

transform domain were recovered along the western ridge transform intersection (WRTI).

A subset of these samples from the WRTI are very different and do not appear to be









related by fractional crystallization to the other Siqueiros samples. Compared to all other

samples from the Siqueiros transform, these RTI samples have higher A1203 and lower

P205, K20, Na20, and CaO contents for a given MgO.

The samples from spreading center B, which exhibits the most symmetric

spreading pattern, were compared to determine whether or not there is symmetry of lava

composition about the axis and to determine if there is a systematic change in lava

chemistry with time (distance from axis) for the intra-transform spreading centers. As

basalts are carried off-axis by spreading, they record the chemical composition of the

axial melt lens at the time they were erupted. If the basalts are not buried beneath

younger, off-axis flows, the distribution of lava compositions may show systematic

differences in magma chamber chemistry with time.

Spreading center B was chosen because it is the most well sampled and it appears

to be the most well developed spreading center in the Siqueiros transform.

Morphological symmetry of ridges can be identified up to 30-40 km from the axis of

spreading. This suggest that spreading center B has been active longer than the other

spreading centers, which only exhibit symmetry 10-20 km from the spreading centers.

Variations in MgO content and depth to seafloor were compared with the sample's

distance from the axis of B (Figure 5-4). Samples recovered from the axis have a wide

range in MgO contents (-7-8.5 wt. %). Most of the samples with the highest MgO

contents were found within the axis, but high MgO samples were also found on the

western side. The most evolved samples were found furthest east from the axis. Smooth

fit lines of depth and MgO variation with distance from the axis show no correlation

between sample depth and MgO content. Based on the samples recovered, there does not









appear to be a symmetrical variation in MgO contents or any systematic change in lava

chemistry with distance or depth, but sampling is too sparse off axis to truly evaluate this.

Comparison of Siqueiros Samples to the Adjacent EPR and Garrett Transform

MORB mantle compositional heterogeneities exist on various scales: global,

regional and local. Local variability can exist on segment and sub-segment scales with

compositional variations along and across axis for individual segments and even within

individual lava flows (Perfit & Chadwick, 1998). In order to better understand global and

regional variability, the major element contents of the Siqueiros samples were compared

to that of the adjacent EPR and the Garrett transform in order to determine how the

samples relate to the regional chemistry of the EPR and the chemistry of lavas erupted in

other fast slipping transforms (Figures 5-5 and 5-6).

The major element compositions of Siqueiros samples can be compared to the well

studied 9-100N segment of the EPR, which is directly north of the transform. Most of the

Siqueiros samples from the 3 intra-transform spreading centers fall within the EPR fields.

Siqueiros samples from the faults are less evolved and slightly more depleted in K20 and

P205 than the EPR samples with the picrites and the picritic basalts from the A-B fault

and some of the samples from the C-D fault being particularly less evolved than the EPR

lavas. A few of the samples from the RTIs fall outside the EPR field and are unlikely to

be related to the EPR samples by fractional crystallization.

Siqueiros samples from the 3 intra-transform spreading centers and transform shear

zones are similar to Garrett transform basalts, but have slightly lower P20s, K20, and

TiO2 contents. Lavas from spreading center A, which were found to be depleted in Na20






74





2000
22000 ------'--------------------------'------------4-----------I--

2400 -.....---- ............... --. ....... ....................... .......... .........

.c 2600 ................... "-................------------- -- ... .....--- --.. .."- ....-.


0 2800 ------- --- ---------- ---------- --A-------
3000 ------------ -- --- ............... --------------------


3200 ---------- ----- -- --.-A-. .. ; ...................... ......... ...........

6.5






9A
7.5 .. .---------- -...... ................ ....................... ..........
8. A...........






9 I I I .
8 -" -- - - . . - -- - --- ,---- -- -- -- -
0 C 0 C) C) C) C) C)


Distance from B axis (m)



Figure 5-4. MgO (wt. %) and depth to seafloor versus distance from the axis of
spreading center B. Negative values are west of the axis and positive values
are east of the axis. Thick line is smooth fit trend.




































0 -


I I I I I I


a
.1 .~ S


3


2.5


2
0
F
1.5


1





51


50

O
W 49


48


47


Q [j

^^t^^J~d


I I I I I I


44
A &


~i~y I~


I I I I


5 6 7 8 9 10 11 5 6 7 8 9 10 11
MgO MgO


Figure 5-5. Variation diagrams comparing Siqueiros lava compositions with basalts from the 9-100N segment of the EPR (Perfit et
al., personal communication).


A Spreading Center A
E A-B Fault
A Spreading Center B
* B-C Fault
A Spreading Center C
* C-D Fault

* Ridge Transform Intersection
EPR Field


A A
* -


3.5




3
Z

O
0

2.5






13



12

0
Q)
11 0



10


Q


-*

































8

7



18

17

016

< 15

14

13

12


i i i i i i


- -

. tU 1 ; L


V


Spreading Center A
A-B Fault
Spreading Center B
B-C Fault
Spreading Center C


* Ridge Transform Intersection
EPR Field


- *


i i I i I I i i


5 6 7 8 9 10 11 5 6 7 8 9
MgO MgO
Figure 5-5. Continued.


i Er i 0

10 11


0.6

0.5

0.4

0.3 N
0
U,
0.2

0.1

0
0.7

0.6

0.5

0.4 r
0
0.3

0.2

0.1

0


**





@#0 E1
i Eli ic









































i I i I i


i I i i I i


Spreading Center A
A-B Fault
Spreading Center B
B-C Fault
Spreading Center C


* Ridge Transform Intersection
Garrett Field


A


I I I I I I


i i I i i i


5 6 7 8 9 10 11 5 6 7 8 9 10 11
MgO MgO

Figure 5-6. Variation diagrams comparing the compositions of the Siqueiros and Garrett samples (from Hekinian et al., 1995).
























13 Ridge Transform Intersection 0.6
Garrett Field
12 -- 0.5

11

10.

LL.


8-
--- ------ ------- --- 0.2

7 O
O 3 010






18 --0.7

17 -- 0.6
-0.5
0"16-
-0.4 "
415-
0.3
4 i- ----- ---14 ----- 0.2
17 ---- ------ --- ---




13 -











5 6 7 8 9 10 11 5 6 7 8 9 10 11
MgO MgO

Figure 5-6. Continued.
0.5

0 A,



0.2



12 0t~
5 6 7 8 9 10 11 5 6 7 8 9 10 11
MgO MgO

Figure 5-6. Continued.









compared to other Siqueiros samples, are also slightly depleted in Na20 compared to the

Garrett samples. On average the RTI samples are more evolved than the Garret samples

and are more enriched in K20 and P205.

Trace Element Trends

The trace element contents of the Siqueiros samples are presented in Appendix D.

Selected trace elements were plotted against TiO2 and Zr, which both behave

incompatibly during crystal fractionation and mantle melting (Figures 5-7 and 5-8).

Incompatible element contents generally increase with increasing fractional

crystallization, while compatible element contents decrease with magmatic evolution.

The trace elements Ni and Cr behave compatibly exhibiting relatively coherent trends

that decrease with increasing TiO2 and Zr. Ni is compatible in olivine and initially

decreases with fractionation. Cr is compatible in spinel and olivine and exhibits a very

high initial decrease with fractionation. Y, V, and Zr all behave incompatibly and

increase smoothly with increasing magma evolution. Sr, which is compatible in

plagioclase, initially increases with increasing TiO2 and Zr, but then levels off increasing

only slightly with further magmatic differentiation.

In general the samples show a narrow range in total abundance for each trace

element except for the RTI samples. Samples 2390-1, 2390-3A, 2390-3B, 2390-4, 2390-

5, and 2390-8 are enriched in the trace element Sr and depleted in Y. Samples 2390-1

and 2390-5 are also slightly enriched in Ni when compared to the other evolved samples.

These samples from the RTI do not appear to be related to the other samples by fractional

crystallization and are classified as E-MORBs based on their Ce/Yb ratios. Samples RC-

41 and D30-1 (both from the ERTI) and sample 2390-9 (from the WRTI) do not group

with the other samples from the RTI, but appear more similar to those from the spreading
























150




100




50


1600

1400

1200

1000

800

600

400

200


0.


A Spreading Center A
o A-B Fault
* Spreading Center B
* B-C Fault
A Spreading Center C


Ridge Transform Intersection

Zr

A




A iA







-Cr
-



















_A A
-I D
-



-^


5


TiO
2

7. Trace elements versus TiO2. Ti behaves incompatibly and increases with
magma evolution. Trace element data from XRF and DCP analysis.


DCP analysis.


"












A Spreading Center A
E A-B Fault
A Spreading Center B
B-C Fault
A Spreading Center C


Ridge Transform Intersection

Ni
800 -


600 -


400 -


200 DE%



64 -

56 -

48 A

40 -

32 -

24 -

16 E

8 -
I I I I

0.5 1 1.5 2 2.5 3
TiO
2


Figure 5-7. Continued.












A Spreading Center A
o A-B Fault
i Spreading Center B
B-C Fault
A Spreading Center C


Ridge Transform Intersection


320 -Sr

280 -

240 -

200 -

160 -

120 A

80 [313 A A ak
80 A i__


0.5 1 1.5 2 2.5 3
TiO
2


Figure 5-7. Continued.

















Spreading Center A
A-B Fault
Spreading Center B
B-C Fault
Spreading Center C


* Ridge Transform Intersection


A *A
A AA
4

A A
A
4 *


I I I I I I I


Cr
0


500

450

400

350

300

250

200




1600


1200


800


400


4
~


40 60 80 100 120
Zr


140 160 180 200


Figure 5-8. Trace elements versus Zr. Zr behaves incompatibly and increases with
magma evolution. Trace element data from XRF and DCP analysis.


- ayA
-*

O O I 1


: ^

















Spreading Center A
A-B Fault
Spreading Center B
B-C Fault
Spreading Center C


Ridge Transform Intersection |


5Ni

El


0


A A






Y


A


.

M 11 A
I-






40 60 80 100 120 140 160 180 200

Zr


Figure 5-8. Continued.














A Spreading Center A
o A-B Fault
A Spreading Center B
* B-C Fault
A Spreading Center C


Ridge Transform Intersection


Sr








A



j ^ I I I I ,


40 60 80 100 120
Zr


140 160 180 200


Figure 5-8. Continued.









centers and faults. Samples from spreading center B show a slightly greater range in

abundance of the trace elements than the samples from the other spreading centers and

faults. A few samples from spreading center A show a slight depletion at given TiO2 and

Zr values in Sr contents relative to other moderately evolved samples found within the

Siqueiros transform.

The relative enrichment factors for the trace elements Nb, Sr, Zr, and Y, which

have been precisely determined by XRF, were computed by dividing the highest value by

the lowest value for each morphotectonic location in the transform domain (Table 5-1).

The most incompatible element (Nb) shows the greatest relative enrichment for each

location with the exception of the C-D fault in which trace element analysis was only

completed for two samples from the fault. Nb and Zr, the most incompatible elements,

show a lower enrichment than Y and Sr in the two C-D samples. This is because the

REE patterns of these samples cross and they cannot be related by fractional

crystallization. Sr, the most compatible element, generally shows the least amount of

enrichment for each location.



Table 5-1. Nb, Sr, Zr, and Y enrichment factors for Siqueiros transform morphotectonic
locations.
Enrichment Factors
Nb Zr Y Sr
Spreading Center A 3.00 2.05 1.63 1.73
A-B Fault 6.59 3.75 3.35 2.74
Spreading Center B 2.72 2.13 2.10 1.53
B-C Fault 2.25 2.33 1.29 1.24
Spreading Center C 3.85 1.66 1.39 1.29
C-D Fault 1.07 1.05 1.24 1.22
Trough D 1.38 1.15 1.05 1.19
Ridge Transform Intersection* 1.79 1.82 1.77 1.13
*E-MORBs not included in calculation of RTI enrichment factors.









As previously discussed, the K20/TiO2 ratio has been used as a proxy for degree of

enrichment based on comparison with (Ce/Yb) ratios. The (Ce/Yb) ratio compares a light

rare earth element (LREE), Ce, to a heavy rare-earth element (HREE), Yb. When

normalized to chondrites a (Ce/Ybn) ratio of 1 indicates no enrichment or depletion of the

LREE to HREE relative to chondrites. A (Ce/Ybn) ratio greater than one indicates that a

sample is LREE enriched and a (Ce/Ybn) ratio less than one indicates LREE depletion.

The (Ce/Ybn) values can be used to determine what K20/TiO2 value matches the

boundary between enriched and depleted samples. The measured K20/TiO2 value of a

sample can then be used to classify samples in which trace element analysis has not been

completed as enriched or depleted. Figure (5-9) shows the (Ce/Ybn) vs. K20/TiO2. All

samples except for the RTI samples are depleted compared to chondrites. Sample 2390-9

has a (Ce/Ybn) ratio about equal to one and samples 2390-1, 2390-5, and 2390-3B are

enriched compared to chondrites. The (Ce/Ybn) break in enriched vs. depleted samples is

roughly at about K20/TiO2 = 0.11. Consequently, most of the Siqueiros samples are

classified as depleted and only a few samples from the RTI show an enriched signature.

The samples from the spreading centers and the faults for the most part fall into

narrow (Ce/Ybn) groups based on their location (Figure 5-10). The samples from

spreading center B and the C-D fault have higher (Ce/Ybn) ratios indicating that they are

more enriched overall than the other Siqueiros samples. Samples from the A-B fault and

spreading center A (except for a few samples) have the most depleted (Ce/Ybn) values.

Of all the samples for which trace element analysis was done only sample 2390-9

exhibits transitional trace element characteristics.